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Thermodynamic Properties

Of Ammonia as an Ideal Gas

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UNITED STATES DEPARTMENT OF COMMERCE

C. R. Smith, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Thermodynamic Properties of Ammonia

as an Ideal Gas

Lester Haar

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

Reprinted from the Journal of Research of the National Bureau of Standards

A. Physics and Chemistry

Vol. 72A, No. 2, March-April 1968

NSRDS-NBS 19

National Standard Reference Data SeriesNational Bureau of Standards 19

Issued August 1968

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tiff ? * f

Foreword

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Ill
Contents

PAGE

Foreword III

1. Introduction 1

2. Partition function 2

3. Molecular data 3

4. Thermodynamic functions 5

5. Discussion 8

6. References 9

IV
Thermodynamic Properties of Ammonia as an Ideal Gas*

Lester Haar

Institute for Basic Standards, National Bureau of Standards, Washington, D.C. 20234

(December 15, 1967)

Thermodynamic functions for ammonia as an ideal gas at one atmosphere pressure have been

evaluated. The contribution of the highly anharmonic out-of-plane vibrational mode, including its large coupling with rotation and its coupling with the other vibrational modes, is considered in detail. Tables of C°PIR, ( H° — E°0 )/RT, ( E°0 — G°)IRT, and S°/R have been calculated at closely spaced inter- vals from 50 to 5000 °K within an overall uncertainty of less than 0.1 percent at 1000 °K. Key Words: Ammonia, ideal gas, thermodynamic functions.

1 . Introduction

This paper is part of a program originating with the

Office of Standard Reference Data of the National

Bureau of Standards to obtain accurate thermodynamic properties for ammonia. Subsequent papers of

this program will include real-gas corrections to the

ideal-gas properties contained herein.

In this paper the ideal-gas thermodynamic properties

of ammonia in the ground electronic state are calculated at closely spaced temperature intervals from

50 to 5000 °K. The calculations include the contributions of ordinary vibrational anharmonicity and vi- brational-rotational coupling together with rotational

stretching and rotational quantum effects. Particular

attention is given to the treatment of the rotational

and vibrational anharmonic effects due to the molecular inversion. To this end a method is developed for

calculating this contribution such that an overall

accuracy for the thermodynamic properties of am- monia approaching that of the simplest diatomic

molecules is obtained.

In the vibrational ground state ammonia is a pyramidal molecule with the nitrogen atom approximately

0.38 A above the plane of the three hydrogen atoms.

The distance between the nitrogen atom and each of

the hydrogen atoms is approximately 1.0 A. Thus the

pyramid is relatively flat and the potential barrier to vibration of the nitrogen atom through the plane of

the hydrogen atoms so as to invert the pyramid is low (approximately 2000 cm-1

[l]). 1 Ammonia has six vibrational degrees of freedom.

However, owing to symmetry there are only four normal

vibrational modes, two of which have double degeneracies. The vibrational structure may thus be char-

*This work was supported in part by the Office of Standard Reference Data, NBS.

1 Figures in brackets indicate the literature references at the end of this paper.

acterized by the six quantum numbers (viv2 v£3v^),

where the Vi are the principal vibrational quantum

numbers associated with the vibrational modes i

n

and the / characterize the small coupling of the degenerate and V4 asymmetric vibrational modes with

the angular momentum of the molecule. The V\ and

v2 modes are symmetric, nondegenerate vibrations

corresponding to stretching the N-H bond and spreading the pyramid, respectively. (The v2 mode is also

referred to as the out-of-plane vibration.) The rotational

structure for the ground state is that of a symmetric

top, so that the rotational energy levels are characterized by the two quantum numbers J and K.

The v2 mode is the vibration associated with the

inversion of the pyramid. Each vibrational state of this

mode is split into two components, corresponding

respectively to symmetric and antisymmetric eigenfunctions. (In the nomenclature these are identified

by the superscript “

5 ” or “a” to the quantum number

v2 .) The spacing of these levels is highly anharmonic

except for vibrational energies sufficiently higher or lower than the potential barrier to inversion. Thus the

vibrational levels for the ground state of the v2 mode

are only slightly split, of the order of a fraction of a wavenumber, but the splitting increases rapidly with

increasing vibrational quantum number. At energy

levels sufficiently elevated above the barrier (greater

than about 3000 cm-1

) the levels become uniformly

spaced with a spacing of approximately half that of

the v2 fundamental. Similarly, due to inversion the

rotational structure undergoes a transition from that

of a pyramid for the low values of the v2 quantum number to that of a planar molecule for the higher values, so that the rotational motion is also strongly coupled

to the v2 mode. For a more detailed discussion of these

effects, see the review by Herzberg [2] of the pyramidal

XY2 molecule and Nielsen [3]. The energy structure

1
of the other vibrational modes is similar to that of

typical, slightly anharmonic rotating vibrators.

In section 2 the equations for the calculations are developed. This is followed in section 3 by a discussion

of the molecular data, and in section 4 by a discussion

of the calculated properties and an evaluation of their

accuracy. Finally, a discussion is given in section 5

of some of the earlier work.

2. Partition Function

It is convenient to represent the rotational and vi- brational structure by an expression for which the

energy levels of the v2 mode and its coupling with

the other degrees of freedom are written explicitly.

Thus the energy levels in wavenumbers are written

T( V, y, K) =G'{v) +F'(v, J , K) +G2 (v, J, K), (1)

where the primes denote that dependence on v2 is omitted. The first term in eq (1) describes the

vibrational energy characterized by (viOvlfv!?) and in

notation similar to that in Herzberg [2] is approximated

by

G'(v) =2 ViVi + '*? XijViVj + ^'xu(vi— l)vi + . . .

i i>j i + ^34/3/4 + ^33(^3 — ^3) + g44 ( l \ — V4 ) + . . . , (2)

i,j^ 2,

where the Vi are fundamentals and the x and g are

relatively small anharmonicities. The second term in

eq (1) is approximated by

F'(vJ , K) = B'J{J + 1) + (C'V —B'V )K2 (3)

-Dt'P(J + l) 2 -DrJU+ I)K2 -DK'K4 +

where Cv is the rotational constant corresponding

to rotation about the symmetry axis and Bv is the constant for axes perpendicular to the symmetry axis.

The quantities DJ

V , DJ

VK

, and D% are small coefficients

associated with centrifugal distortion of the molecule. The subscript v to each of the coefficients of

eq (3) indicate dependence or coupling with the

vibrational motion. The primes indicate that dependence on V2 is omitted; that is, the rotational

constants of eq (3) are evaluated for v2 equal to zero. The coupling of the rotational motion to vibrations

other than the v2 mode is relatively small and is included here only to first order in the fundamentals.

B'V = B0 — ^' afvi +

i

C'v= Co- ac

t Vi + , i #2

i

where B0 and Co are rotational constants, and the a

are small corrections. All coordinates involving

the v2 mode, including its coupling with the other

degrees of freedom are included in G2 . Thus G2 includes those effects characterized by

{vx v2 v

lJv\4JK) — (vfivIfv^JK).

The partition function for the rotational and vi- brational structure may be written

Q= 2 exp{-T(v,J, K)c2 /T}, (4)

vJ,K

where T(v, J, K) is discussed in eqs (1), (2), and

(3)

. C2 is the second radiation constant and T is the

absolute temperature. The sum extends over all rotational and vibrational degrees of freedom in- cluding all values of the v, J , K quantum numbers.

In the evaluation of eq (4) the contributions of the

vibrational anharmonicity and vibration-rotation

coupling effects contained in eqs (2) and (3) are

all to be accounted for to first order; that is, only

terms linear in x, g, and a are retained. Though

G2 is not conveniently described by the usual power

series in the vibrational quantum numbers, its contribution, including coupling effects with the other

degrees of freedom, is also to be included to this order.

The contribution of the G2 term is obtained by ex- panding the partition function about the term-byterm sum of the observed (Ot^OO) energy levels. This

summation incorporates the major effect of the highly

anharmonic v2 mode, which is the coupling between

the symmetric and antisymmetric states, but it does

not include the contribution of the v2 coupling with the

other vibrational modes. From an examination of the

spectroscopic vibrational data (see Benedict et al.

[4]

), it can be seen that this cross coupling produces a

shifting of energy levels of about the same magnitude

as that of the other vibrational couplings (those represented by the x and g terms in eq (2)). Thus the contribution to the partition function of this cross coupling

should be of the same order as that from the other

anharmonicities, and a first order approximation for

this contribution should be adequate.

In the expansion that follows expressions for the cross coupling are developed and their contribution

for each individual (Oi^OO) state obtained explicitly.

The cross coupling between a pair of vibrational states,

e.g., states corresponding to the particular quantum

numbers V\ and v2 , is defined as GVl v2 — G(viVzOO) — G(yiOOO) — G(0i>200) , (5)

where G(viV2vfcvlf) is the vibrational energy. A useful

representation for the coupling involving the highly

anharmonic v2 mode is the development of GVll>2 as a power series of products of the observed energies,

that is

(*viv2 ~ ki 2 G (

1^ 1 000 )

• G(0t>200) F .... (6) The constant k\ 2 is readily determined in terms of the

observed coupling. Thus for coupling between the

fundamentals the observed coupling xn is given by

2
*i2 = Jti 2 C(1000) • G(0100) + . . . = ki 2 uiv2 + ....

Note that eq (6) reduces identically to the power series

of eq (2) when the cross coupling is between nearly

harmonic modes.

Consider the energy of the nearly harmonic oscillator vm , neglecting all anharmonicities but the cross coupling with v2 . The energy levels for such an oscillator are given by

Gm VmVm d

- GVmV^.

Substituting from eq (6) and defining E2 = G(0z;200),

this becomes

The prime indicates that the m = 2 factor in the product

corresponding to self coupling of the v> mode is omitted.

The coupling of rotation with the v2 mode is in- cluded by evaluating the classical rotational partition

function for each of the (CU2OO) energy levels and employing these as separate weighting factors to each of

the terms in eq (12). These factors are the usual ex- pression for a classical nonrigid rotator,

Qk{v2) =771/2 (cr!crc)~ 1/2 (l + p

{1) T +p(2) r

2 ). (13)

The quantities crB and <Jc are given by.

Gm — VmVm “1“ k2 mVmVm ' E2. (7)

The contribution of Gm to the partition function is the

factor

Qm = \ 2 exp — [ (C2 IT) {VmVm + klmE-zVmVm)] ( 8)

where dm is the degeneracy of the rath mode. The

anharmonicity term in the exponential of eq (8) is now

expanded and only the linear term retained to yield

Qm =

C2 VmVm 1 ij: kzmVmVmEz

( 9)

Using the identity

d_

da 2 ve~

the contribution of Qm becomes

<?m =(l-e-r (10)

where

=

cz VC r

J1

l?2 ’

where BV2 and CV2 are the observed rotational constants for the particular (Ot^OO) energy level. The factor

1 + p

(1) T+

p

(2) T

2 is the contribution of rotational

stretching (see, for example (5 and 14)), where the p

are calculated from the rotational stretching coefficients that apply to the particular (0y200) level. One further simplification is required in order to evaluate the out-of-plane contribution. This is to obtain an approximation for the contribution of the

energy levels corresponding to high values of the v2 quantum number. Note that the v2 mode is nearly

harmonic for energies corresponding to the fourth

quantum number and higher, so that the contribution

of these levels may be replaced by the well-known

expression for a slightly anharmonic rotating vibrator.

Combining eqs (12) and (13) and including the closed

form approximation for the contribution of the states corresponding to quantum numbers given by v> > 3

yields the expression for G> (y, K) used in this

calculation.

<?-={2|(§)

, 'V+P">r+^»r).

j

J /2m=4yfe/2^/(eWra/T -l). (11) exp ( — c->E

4 sIT

)

Bo/Co\ 1/2

1 — exp (— czv'zIT) B4 \Cy

The factor (1 —f2m) is the contribution of the interaction of a particular (Ot^OO) energy state with the

rath vibrational mode, and it depends explicitly on E2 . It is clear then that the effect of this coupling is to modify each of the terms in the term-by-term sum

over the ((WX)) energy levels by the factor (1 —f2m )• The term-by-term sum including cross coupling with

the other vibrational modes is thus given by

QV2 (vib)= 2) FI 0 ~hm) exp (—c2E2 /T). (12) E2 m

[] (1 — 02j) (1 + (f>2 ) (l + p

{1) T) , (14)

where E = E2 and the subscript to E now refers to the

particular v2 quantum number. The quantity in the

braces in eq (14) includes levels only up to t>2 = 3

a

. Note that the rigid rotational contribution for the

ground state has been factored from eq (14). In the

0xp (

c>E JJ

1 ) second term the factor ^ 7 4

, ,

is the har1 — exp (— C2 v2 l 1 ) monic oscillator contribution with EaS as the first level

289-015 0-68—7 3
and v2 the fundamental. The factor 1 — 02j is the ordi- 3. Molecular Data

nary anharmonicity coupling with the yth vibrational

mode, where

Oij p

Xjjdjdj

(exp (c-’Vi/T) — 1) (exp (c2 VjlT) — 1)'

Y

(xu — gal3 ) di ( di + 1

)

(exp ( czVi/T

)

— l) 2 (15a)

The factor 1 + f>i is the well-known contribution of

rotation-vibration coupling, where

4>i = di (a? 1 «F\

\B 2 C) (15b)

Details concerning the derivation of quantities in eqs

(15) may be found in Friedman and Haar [6], Woolley

[7], and Pennington and Kobe [8], for example.

The contributions to the partition function of the

two remaining terms G' ( v , J , K) and F' (v, J , K) are

quite straightforward (see [6, 7, 8]). With these the

complete partition function is finally obtained.

Mirm=

1

(i-«„) [{' (1 + <M

isj A:=l

' Qtr ‘ Qqr (16)

As noted earher the primes on the product summations

indicate that factors containing the vibrational coordinate 1)2 are omitted. The quantity S is the symmetry

number. For the usual symmetric top molecule this has

the value three. However, each of the components of

the nondegenerate but split (Ot^OO) levels have been

counted with a statistical weight of unity, so that the

factor QV2 is larger by a factor of two. This error is cor- rected by setting S = 6. (Incidentally S — 6 corre- sponds to the symmetry for a planar configuration for

NH3 , and eq (16) might be thought of as an expansion

about this planar configuration.) The product over m is the harmonic oscillator contribution for the modes

other than v2 ; the products over ij and over k are the

corresponding anharmonicity and rotation-vibration

coupling contributions, respectively. The degeneracy

coefficients d are unity for m= 1, 2, and two for ra = 3, 4. The Oij and <\>k are given by eq (15). Qtr is the usual

contribution of translation. Lastly, the factor Qqr is a correction for rotational quantum effects. It is very

small at temperatures for which the vibrations are appreciably excited, so that it is not coupled to the v2 mode. The contribution Qqr is given by Stripp and

Kirkwood [9] for an asymmetric rotator, which for a symmetric top rotator reduces to ob

Qqr — 1 + ^ _ _| | B 4 crc) 15

I -

4 o-c 32

The molecular data upon which the calculations are based are fisted in tables 1 and 2. Table 1 fists the data

sensitive to the inversion coordinates and utilized in

the term-by-term sum (the quantity in braces in eq (14)).

The vibrational energies corresponding to i>2 = 0, 3 are from [4]. The energies corresponding to V2 — 1, 2 are from Garing et al. [10]. The values fisted for the rota- tional constants Bv and Cv corresponding to the

i>2 = 0 states are from [4]; those corresponding to the

V2 — 1, 2 states from [10]; and to the V2 = S states from

Benedict et al. [11]. The rotational stretching constants

D{, DJ

VK

, D$ are the average of the symmetric and antisymmetric values. The values for i>2 = 0 are from [4]; for V2 — 1, from [10]; for i> 2 = 3, from [11]; finally, data

for i)2 = 2 were taken equal to the V2 —I values.

Table 1. The out-of-plane molecular constants in cm~ 1

, \2 3, (OV2 OO)

State O' 0° 1* 1° CN CVJ 3* 3«

v2 0.0 0.793 932.41 968.04 1597.42 1882.16 2383.46 2895.48

Br, 9.9443 9.939 10.07 9.89 10.26 9.73 9.50 9.20

Cv2 6.196 6.198 6.05 6.13 5.90 6.13 6.16 6.26 &c2 8.1 x 10-' 8.9 x 10-* 8.9 x 10-' -3.3x10-'

pjK - 14.5 x 10-* - 17.5 x 10-4 — 17.5 x 10"* 15.0x10-* DK 7.5 x 10-" 10.5 X10-" 10.5 x 10-' -11.3X 10-'

Table 2. Vibrational, rotational and coupling constants in cm-1

; see eqs (2), (3)

t/,(l 0 0 0) v't =Eia —Et, i/3(0 0 1 0) j/4(0 0 0 1)

3323.06 475 3443.381 1627.77

O

II 1 CO H *22 = +

9

*33 = — 18.5 *44 = 8.8

*12 = + 20.6 *23 ~ + 32.36 *34 = — 17.3 £44 =+ 2.65

*.3 = -92 *24=- 10.7

*n =— 6.7 B, = 9.35, C4 = 6.21

a? = 0.135 of = + 0.015 of =-F0.176 of =-0.230 af= .078 a£ = + 0.098 aj = — 0.009 af = + 0.066

The remaining molecular data are given in table 2. The vibrational fundamental Vi is from Benedict

et al. [12]; is from [11]; V4 is from [10]. The constants

i^and X22 which correspond to the high energy, nearlyharmonic region of the V2 mode are derived from the

reported values for the (04*00) and (05*00) levels at 3330 cm-1 and 4315 cm-1 given by Walsh and Warsop

[13] and from theoretical considerations that at high

energies the spacing between the levels should become

nearly equal to one-half the (0100) value. Also pertaining to this region are the rotational constants B4 and C4 which are taken equal to the averages of the values for

f2 = 3* and 3

a

. The anharmonicity coefficients X 12 , X13 , X14 , X23 , X24 , X33 , and X34 are from [4], and the coefficients X44 and g44 are from [10]. The anharmonicity

coefficient xn is estimated from data for similar

4
stretching vibrations. The coupling constants af

are from [4]. The small coefficient gw is taken as zero. The values used for the atomic masses are from

Cameron and Wichers [35J. The values used for the

other physical constants are given in [36].

4. Thermodynamic Functions

The thermodynamic properties of ammonia as an

ideal gas at a pressure of one atmosphere are listed in

table 3. These include the Gibbs (free) energy function,

enthalpy function, heat capacity at constant pressure,

and entropy tabulated in dimensionless units at closely

spaced temperature intervals from 50 to 5000 °K.

Should it be desired to obtain the properties for a pres- sure of 1 bar (106 dynes cm-2 or 105 Nm~2

) the magnitude of the tabulated free energy and entropy values

should be increased by 0.0132. Conversion factors to other units in frequent use are given in table 4. The

entries in table 3 are truncated at four decimals (except for C°p/R above 2500 °K which is truncated at three) but at best they are accurate to three decimals.

It is felt that the additional significance could be useful

in applications involving differences between the

entries. The density of entries permits use of linear

interpolation throughout the table with a resulting

interpolation error smaller than the uncertainty in the

tabulated values.

As indicated by the earlier discussion the present

calculation, except for the out-of-plane vibrations, is based on an expansion about the harmonic oscillatorrigid rotator model. In this expansion only first order

(linear) terms in the coupling between the various

degrees of freedom are retained. In addition to the

errors inherent in the derivation of the partition function, eq (16), there are also errors due to uncertainties

in the molecular constants, errors which arise from

certain resonances between several of the low-lying

vibrational states, the effects of which are omitted, and

lastly errors arising from omission of excited electronic

states.

Table 3. Thermodynamic functions for ammonia

The tabulated values are spaced sufficiently close to permit linear interpolation between the entries with a resulting error less than

their overall uncertainty. For convenience the values at 273.15 and 298.15 °K have also been included.

T

t*a 0

0 1 (H°-E°0)/RT Cp/R S°/R

50. 12.0017 3.9539 4.0029 15.9555

60. 12.7234 3.9620 4.0026 16.6854

70. 13.3346 3.9678 4.0026 17.3024

80. 13.8648 3.9721 4.0027 17.8369

90. 14.3328 3.9756 4.0029 18.3084

100. 14.7519 3.9783 4.0033 18.7302

110. 15.1311 3.9806 4.0039 19.1117

120. 15.4776 3.9826 4.0051 19.4602

130. 15.7964 3.9844 4.0069 19.7808

140. 16.0918 3.9861 4.0097 20.0779

150. 16.3668 3.9878 4.0138 20.3546

160. 16.6243 3.9896 4.0194 20.6138

170. 16.8662 3.9915 4.0267 20.8577

180. 17.0944 3.9937 4.0358 21.0881

190. 17.3104 3.9962 4.0468 21.3066

200. 17.5154 3.9991 4.0599 21.5145

210. 17.7106 4.0023 4.0749 21.7129

220. 17.8969 4.0060 4.0920 21.9029

230. 18.0751 4.0101 4.1110 22.0852

240. 18.2458 4.0147 4.1319 22.2606

250. 18.4098 4.0199 4.1548 22.4297

260. 18.5676 4.0255 4.1794 22.5931

270. 18.7196 4.0317 4.2057 22.7513

273.15 18.7663 4.0338 4.2143 22.8001

280. 18.8664 4.0384 4.2336 22.9048

290. 19.0082 4.0456 4.2630 23.0539

298.15 19.1204 4.0520 4.2882 23.1724

300. 19.1455 4.0534 4.2939 23.1989

310. 19.2785 4.0617 4.3260 23.3402

320. 19.4076 4.0704 4.3593 23.4781

330. 19.5330 4.0797 4.3937 23.6127

340. 19.6550 4.0895 4.4291 23.7444

350. 19.7737 4.0997 4.4654 23.8733

360. 19.8893 4.1104 4.5025 23.9997

370. 20.0021 4.1215 4.5402 24.1235

5
Table 3. Thermodynamic functions for ammonia — Continued

T (E°0 -C)/RT (H°-E°t)IRT C°JR S°/R

380. 20.1121 4.1330 4.5785 24.2451

390. 20.2196 4.1449 4.6173 24.3646

400. 20.3247 4.1572 4.6565 24.4819

410. 20.4275 4.1699 4.6961 24.5974

420. 20.5282 4.1829 4.7359 24.7111

430. 20.6268 4.1962 4.7759 24.8230

440. 20.7234 4.2098 4.8161 24.9332

450. 20.8181 4.2237 4.8563 25.0419

460. 20.9111 4.2379 4.8966 25.1491

470. 21.0024 4.2524 4.9368 25.2548

480. 21.0921 4.2671 4.9771 25.3592

490. 21.1802 4.2820 5.0172 25.4622

500. 21.2669 4.2971 5.0572 25.5640

520. 21.4360 4.3278 5.1368 25.7639

540. 21.6000 4.3593 5.2158 25.9592

560. 21.7591 4.3912 5.2939 26.1503

580. 21.9137 4.4237 5.3712 26.3374

600. 22.0643 4.4566 5.4475 26.5208

620. 22.2109 4.4898 5.5230 26.7007

640. 22.3540 4.5232 5.5975 26.8772

660. 22.4937 4.5569 5.6711 27.0506

680. 22.6302 4.5907 5.7438 27.2210

700. 22.7638 4.6247 5.8157 27.3885

720. 22.8946 4.6588 5.8867 27.5533

740. 23.0227 4.6929 5.9568 27.7156

760. 23.1483 4.7271 6.0261 27.8754

780. 23.2715 4.7613 6.0946 28.0328

800. 23.3925 4.7954 6.1622 28.1879

820. 23.5113 4.8296 6.2291 28.3409

840. 23.6281 4.8637 6.2952 28.4918

860. 23.7430 4.8978 6.3604 28.6407

880. 23,8559 4.9317 6.4249 28.7877

900. 23.9672 4.9656 6.4885 28.9328

920. 24.0767 4.9994 6.5513 29.0761

940. 24.1845 5.0331 6.6133 29.2176

960. 24.2909 5.0667 6.6745 29.3575

980. 24.3957 5.1001 6.7348 29.4958

1000. 24.4990 5.1334 6.7943 29.6324

1020. 24.6010 5.1665 6.8530 29.7676

1040. 24.7017 5.1995 6.9108 29.9012

1060. 24.8010 5.2323 6.9678 30.0334

1080. 24.8991 5.2650 7.0239 30.1641

1100. 24.9960 5.2975 7.0792 30.2935

1120. 25.0918 5.3298 7.1335 30.4216

1140. 25.1864 5.3619 7.1870 30.5483

1160. ‘ 25.2799 5.3938 7.2397 30.6738

1180. 25.3724 5.4256 7.2914 30.7980

1200. 25.4639 5.4571 7.3424 30.9209

1220. 25.5543 5.4884 7.3924 31.0427

1240. 25.6438 5.5195 7.4416 31.1633

1260. 25.7324 5.5504 7.4900 31.2828

1280. 25.8200 5.5811 7.5375 31.4011

1300. 25.9068 5.6115 7.5841 31.5183

1320. 25.9927 5.6418 7.6299 31.6345

1340. 26.0778 5.6718 7.6749 31.7495

1360. 26.1620 5.7016 7.7190 31.8636

1380. 26.2455 5.7311 7.7624 31.9766

1400. 26.3281 5.7604 7.8049 32.0886

1420. 26.4101 5.7895 7.8466 32.1996

1440. 26.4912 5.8184 7.8875 32.3096

6
Table 3. Thermodynamic functions for ammonia — Continued

T (E°0 -C)IRT (.H°-E%)IRT CUR S°/R

1460. 26.5717 5.8470 7.9277 32.4187

1480. 26.6514 5.8754 7.9671 32.5268

1500. 26.7305 5.9035 8.0058 32.6340

1550. 26.9252 5.9729 8.0992 32.8981

1600. 27.1159 6.0407 8.1881 33.1566

1650. 27.3028 6.1071 8.2728 33.4099

1700. 27.4861 6.1720 8.3532 33.6581

1750. 27.6659 6.2354 8.4298 33.9013

1800. 27.8424 6.2974 8.5027 34.1398

1850. 28.0158 6.3579 8.5719 34.3737

1900. 28.1862 6.4171 8.6378 34.6032

1950. 28.3536 6.4748 8.7004 34.8284

2000. 28.5182 6.5312 8.7599 35.0494

2050. 28.6802 6.5863 8.8164 35.2664

2100. 28.8395 6.6400 8.8702 35.4795

2150. 28.9964 6.6925 8.9214 35.6889

2200. 29.1508 6.7437 8.9701 35.8945

2250. 29.3030 6.7937 9.0162 36.0966

2300. 29.4528 6.8425 9.0602 36.2953

2350. 29.6005 6.8901 9.1021 36.4906

2400. 29.7460 6.9366 9.1418 36.6826

2450. 29.8895 6.9820 9.1797 36.8715

2500. 30.0310 7.0263 9.216 37.0573

2550. 30.1706 7.0696 9.250 37.2402

2600. 30.3083 7.1118 9.282 37.4201

2650. 30.4441 7.1531 9.313 37.5972

2700. 30.5782 7.1933 9.342 37.7716

2750. 30.7106 7.2327 9.370 37.9432

2800. 30.8413 7.2711 9.397 38.1123

2850. 30.9703 7.3086 9.422 38.2789

2900. 31.0977 7.3452 9.445 38.4429

2950. 31.2236 7.3810 9.468 38.6046

3000. 31.3479 7.4160 9.490 38.7639

3100. 31.5922 7.4835 9.529 39.0757

3200. 31.8308 7.5480 9.564 39.3788

3300. 32.0640 7.6096 9.596 39.6736

3400. 32.2921 7.6684 9.623 39.9605

3500. 32.5152 7.7246 9.648 40.2398

3600. 32.7336 7.7783 9.670 40.5119

3700. 32.9474 7.8297 9.688 40.7771

3800. 33.1568 7.8788 9.704 41.0357

3900. 33.3621 7.9258 9.717 41.2879

4000. 33.5633 7.9707 9.728 41.5341

4100. 33.7607 8.0137 9.737 41.7744

4200. 33.9543 8.0548 9.743 42.0091

4300. 34.1443 8.0941 9.748 42.2384

4400. 34.3308 8.1317 9.750 42.4626

4500. 34.5140 8.1677 9.751 42.6817

4600. 34.6939 8.2021 9.749 42.8960

4700. 34.8706 8.2350 9.746 43.1056

4800. 35.0443 8.2664 9.741 43.3107

4900. 35.2151 8.2964 9.734 43.5115

5000. 35.3833 8.3253 9.725 43.7086

The expansion about the harmonic oscillator limit

is essentially a low temperature expansion, and the

errors resulting therefrom for a particular vibrational

mode are small provided jis large, and are of the

order of about the square of the linear term. The

7
Table 4

Conversion factors

to convert tabulated value

to quantity having dimensions

indicated below

Multiply by

Cal mole-1 °K-1 1.98717 Cal g

-1 °K-1 0.116682

Joules mole-1 °K-1 8.3143

Joules g

-1 °K-1 0.488197

largest error of this type occurs for the v4 mode. Thus

for the most sensitive function, the heat capacity, the

uncertainty at 2000 °K is less than 10-2 dimensionless

units, but increases to the order of 0.1 at 5000 °K. The

higher order anharmonicities would contribute an uncertainty perhaps as large. Though the anharmonicity

coefficients Xn and X13 have large uncertainties, the

errors introduced by these are mitigated by the fact

that the fundamentals v\ and V3 are large. A 50 percent

uncertainty in xn and X13 results in an error of about

0.05 dimensionless units in Cp/R at 5000 °K. Setting

g33 equal to zero introduces negligible error since this

coefficient introduces a small correction to an already

small anharmonic contribution (owing to the large

value of the V3 fundamental). The remaining anharmonicities (other than the *22 value in table 2) are probably accurate to better than several percent, and

the uncertainties resulting from these are comparatively small. Uncertainties arising from errors in the

rotational fundamentals are also comparatively small,

as are uncertainties resulting from the use of estimates

for some of the rotational stretching constants. The

uncertainties that arise from errors in the rotationvibration coupling constants are perhaps half as large

as those arising from errors in the anharmonicity

coefficients.

The uncertainties in the values used for the fundamental frequencies are Jess than several tenths of

a wavenumber, and the uncertainty from this source

is negligible. Also negligible are the uncertainties due

to errors in the observed overtones of the r>2 fundamental.

It is somewhat more difficult to determine the uncertainty resulting from estimates used for the molecular constants in the second term in eq (14). The

largest uncertainty here arises from possible errors in

v'2 and E\? . An uncertainty of several percent in these

results in an uncertainty in Cp/R of less than 0.002

dimensionless units at 1000 °K, which increases to a maximum near 2000 °K of less than 0.01 dimensionless

units. The errors from estimates of the vibrational

coupling constants used in this term are completely

negligible below 2000 °K. Even if a 100 percent uncertainty in their contribution is assumed, the resulting

uncertainty at 5000 °K still would be less than 0.1 dimensionless units in C

0P/R.

The spectrum of ammonia is further complicated

by several large resonances that occur between some

of the low lying vibrational bands, such as the reso- nance between the combination bands (2^4 + t^) and

(vi + ^3) and between the bands 2v4 and v\ as discussed

in [4]. Since the resonances tend to displace the re- spective bands symmetrically about their unperturbed

values, it can be shown that the errors are less than

the neglected higher order anharmonicities. A similar

type of error occurs in the treatment of the coupling

of the V2 mode with the other modes. The coupling constants as reported are the average of the symmetric and

antisymmetric values. Thus the effect of this approximation is to introduce errors similar to those arising

from neglect of the resonances discussed above. The

treatment of centrifugal distortion, which includes a quadratic term in the temperature, is consistent with

the other approximations and the error from this source at 5000 °K is probably less than 0.05 in Cp/R.

At the low temperature end there is a small error

arising from the rotational quantum approximation ()qr (eq (17)). At 100 °K this error is less than 10-3 in Cp/R,

but is appreciably larger at 50 °K.

Finally, the U.V. absorption spectrum indicates

bands extending from ~ 46,000 cm-1 to over 100,000

cm-1

, as discussed, for example, in [13]. The error from omission of these even at the very highest temperatures considered here is completely negligible.

It is felt, therefore, that the uncertainty in the tables

in the temperature region 100 to 1000 °K is probably

less than 0.003 dimensionless units in Cp/R. This

uncertainty increases to 0.02 at 2000 °K and then in- creases very rapidly to perhaps 0.3 at 5000 °K. A more

complete list of the uncertainty estimates, including

those for the enthalpy and entropy, are given in table 5.

Table 5. Estimates of uncertainty in dimensionless units

T AC°PIR A (H°-E°0 )IRT AS°/R

100 0.003 0.002 0.003 1000 .003 .002 .003 2000 .02 .01 .02 3000 .05 .02 .03 4000 .1 .05 .07 5000 .3 .1 .2

5. Discussion

Ideal-gas calculations for NH3 have been reported

in the literature since the early thirties, but many of

these calculations were based on relatively incomplete

and inaccurate molecular data and on simplified struc- tural models. Also, for the most part, the temperature

ranges were somewhat limited. The references

[17-27] are representative of these early efforts. Some of the more recent calculations which have

been widely used include those of Harrison and Kobe

[28] who calculated the heat capacity, enthalpy, and

entropy from 273.16 to 1500 °K. They included the

major effects associated with the molecular inversion

by summing the lower states of the v2 mode term by

term, but omitted all vibration and rotational coupling

effects other than rotational stretching. The review by

Davies reported in the monograph edited by Din [29]

8
includes the specific heat, entropy, and enthalpy from

200 to 1000 °K. The calculation follows essentially the

procedure in [28], except that centrifugal stretching

is omitted. Also it appears that the contribution of the

states in the term-by-term sum of the v2 mode was

omitted above the level 3

a

. The most detailed of the

previous calculations is that reported by Yungman

et al. [30], (which also appears in [16J), in which the free

energy, entropy, and enthalpy are tabulated at 100 °K

intervals from 298.15 to 6000 °K. In addition to the

term-by-term sum for the v2 mode, this work also in- cludes several of the first order vibrational and rota- tional coupling contributions. It omits, however, the

contributions due to coupling of the v2 mode with the

.other vibrational modes, and includes a somewhat

simplified coupling of the v2 mode with rotation. These

calculations are based on essentially the same molecular data as those reported here, except that the

values used for the vibrational coupling constants *i3 and *44 are about twice as large. One of the most widely

used set of tables is that of JANAF [31]. The reference

refers to the latest revision for ammonia, September 30,

1965. This calculation follows the procedures of [28]

and [30], though the specific details are not given in

the text. Finally, there are several recent calculations

based on somewhat simplified molecular models,

which are included in references [32-34].

The figures 1 and 2 include a comparison of some

of the previous calculations with those made here. The

ordinates are the deviations from the present calculations, that is the present calculation minus the others.

Figure 1 is a plot of entropy differences, and figure 2

is a plot of heat capacity differences. It is most interesting to compare the data of Yungman et al. [30],

AS

R

0 I 2 3 4 5 TxlO3

FIGURE 1. Dimensionless entropy differences.

Figure 2. Dimensionless heat capacity differences.

curve #5 in figure 1. At the low temperatures the

agreement is quite good, but as the temperature is increased, a positive deviation occurs until about

3000 °K, when the deviation becomes strongly negative.

The difference at the lower temperatures is probably

due to their simplified treatment of the coupling of the

v2 mode with rotation. At temperatures above 1000 °K,

the dominant contribution to the difference is from

the *24 term omitted in [30] which yields a positive

contribution. At temperatures above 3000 °K, the

dominant contribution is from the omitted X\ 2 and *23 terms which yield large negative contributions. In

addition to these, the high value they used for *44 (larger by a factor of two) tends to reduce the differ- ences at temperatures below 3000 °K, but magnifies

them above. The same general comments apply to the

JANAF [31] comparison as seen in curve #1, except

that the difference goes negative at somewhat higher

temperatures and the positive deviations are considerably larger. Incidentally, the JANAF and Harrison and

Kobe [28 1 (curve #4) calculations are nearly identical

for temperatures up to 1000 °K. Curve #3 applies to a modified rigid rotator harmonic oscillator calculation

as given in an older (now superseded) JANAF table.

The comparison for curve #2 [29 1 illustrates the sensitivity to the omission of the hierarchy of states corresponding to large v2 quantum numbers ( v2 > 3). The differences plotted in figure 2 exhibit essentially

the same behavior as those in figure 1 except that they

are somewhat larger.

It is a pleasure to thank Joseph Hilsenrath for many

helpful technical discussions, particularly relating

to the use of OMNITAB to program the calculation. In this latter connection, I also wish to thank Robert C.

Thompson.

6. References

[1] J. D. Swalen and J. A. Ibers, J. Chem. Phvs. 36, 1914 (1962).

[2] G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand & Co., New York (1945).

[3] H. H. Nielsen, Revs. Modern Phys. 23 , 90 (1951).

[4] W. S. Benedict and E. K. Plyler, Can. J. Phys. 35, 1235 (1957).

[5] R. S. McDowell, J. Chem. Phys. 39, 526 (1963).

[6] A. S. Friedman and L. Haar, J. Chem. Phys. 22, 2051 (1954).

[7] H. W. Woolley, J. Res. NBS 56, 105 (1956).

[8] R. E. Pennington and K. A. Kobe, J. Chem. Phys. 22, 1442

(1954).

[9] K. F. Stripp and J. G. Kirkwood, J. Chem. Phys. 19, 1131 (1951).

[10] J. S. Garing, H. H. Nielsen, and K. N. Rao, J. Mol. Sped. 3,

496 (1959).

[11] W. S. Benedict, E. K. Plyler, and E. D. Tidwell, J. Chem. Phys.

29, 829 (1958).

[12] W. S. Benedict, E. K. Plyler, and E. D. Tidwell, J. Chem. Phys.

32, 32 (1960).

[13] A. D. Walsh and P. A. Warsop, Trans. Faraday Soc. 57, 345

(1961).

[14] Y. S. Yungman, Zhur. Fiz. Khim. 35, 319 (1961).

[ 15 j I. N. Godnev, Zhur. Fiz. Khim. 27, 1702 (1953).

[16] V. P. Glushko, Ed., Termodinamicheskie Svoistva Individ- ual’nykh Vestchestv, II, Acad. Sci., USSR, Inst, of Applied

Chemistry, Moscow (1962).

[17] D. S. Villars, Phys. Rev. 38, 1552 (1931); J. Am. Chem. Soc. 53, 2006 (1931).

[18] M. Trautz, Ann. Physik 9 , 465 (1931).

9
[19] F. A. F. Schmidt, Forsch. Arb. Ing. Wes. 8, 91 (1937).

[20] R. Overstreet and W. F. Giauque, J. Am. Chem. Soc. 59 , 254

(1937).

[21] K. K. Kelley, U.S. Bur. Mines, Bull. 407 (1937).

[22] E. Justi, Spez-Warme, Enthalpie und Dissociation Technischer

Gase, J. Springer, Berlin (1938).

[23] C. C. Stephenson and H. O. McMahan, J. Am. Chem. Soc. 61, 437 (1939).

[24] K. S. Pitzer, J. Chem. Phys. 7, 251 (1939).

[25] R. F. Haupt and E. Teller, J. Chem. Phys. 7, 925 (1939).

[26] H. Zeise, Z. Elektrochem. 47, 380 (1941).

[27] H. W. Thomson, Trans. Faraday Soc. 37, 344 (1941).

[28] R. H. Harrison and K. A. Kobe, Chem. Eng. Progress 49, 351

(1953).

[29] F. Din, Thermodynamic Functions of Gases 1 (Butterworths,

London, 1956).

[30] Y. S. Yungman, L. V. Gurvich, and N. P. Rtischeva, Tr. Gos.

Inst. Prikl. Khim. 49 , 26 (1962).

[31] JANAF Thermochemical Tables, Distrib. by Clearinghouse for Federal Scientific Information, PB168370-1, Sept. 30, 1965.

[32] C. L. Mader, Ideal Gas Thermodynamics of Detonation Products, U.S. A.E.C., AECU-4508 (1959).

[33] V. N. Khlebnikova and V. P. Morozov, Ukrain. Khim. Zhur. 24, 3 (1958).

[34] S. Sundaram, F. Suszek, and F. F. Cleveland, J. Chem. Phys.

32 , 251 (1960).

[35] A. E. Cameron and E. Wichers, J. Am. Chem. Soc. 84, 4192

(1962).

[36] NBS Tech. News Bull. 47, No. 10 (Oct. 1963).

(Paper 72A2-495).

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