# Millimeter-Wave Rotational Spectra oftrans-Acrolein (Propenal) (CH2CHCOH): A DC Discharge Product of Allyl Alcohol (CH2CHCH2OH) Vapor and DFT Calculation

Authors: A. I. Jaman; Rangana Bhattacharya
Journal: Journal of Atomic, Molecular, and Optical Physics
 (2012)

---

## Abstract

Millimeter-wave rotational spectrum oftrans-acrolein (propenal) (CH2CHCOH) produced by applying a DC glow discharge through a low-pressure (~10–20 mTorr) flow of allyl alcohol (CH2CHCH2OH) vapor has been observed in the ground and several excited torsional states in the frequency region: 60.0–99.0 GHz. A least-square analysis of the measured and previously reported rotational transition frequencies has produced a set of rotational and centrifugal distortion constants for the ground as well as excited torsional states. Detailed DFT calculations were also carried out with various functional and basis sets to evaluate the spectroscopic constants, dipole moment, and various structural parameters of the trans conformer of propenal for the ground state and compared with their corresponding experimental values. A linear variation of the inertia defect values with torsional quantum number   (v=0,1,2,3) demonstrates that the equilibrium configuration of trans-propenal is planar.


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## Body

## 1. Introduction

Thetransform of propenal (CH2CHCOH) also known as trans-acrolein has been detected largely in absorption toward the star-forming region Sagittarius B2(N) by Hollis et al. [2] through the observation of rotational transitions using 100 m Green Bank Telescope (GBT) operating in the frequency range from 18.0 GHz to 26.0 GHz. Spectroscopic measurements in the microwave [1, 3], infrared [4], and near ultraviolet region [5, 6] have confirmed that the trans-form is the most abundant and stable conformer of acrolein. The first microwave study of trans-acrolein in the J = 2←1, 3←2, and 4←3 a-type R-branch transitions was reported by Wagner et al. [3]. Later on, Cherniak and Costain [1] have measured both a- and b-type transitions for J = 2←1 and J = 3←2. First spectroscopic evidence of the existence of the less abundant cis-conformer of acrolein in the gas phase was found from studies of the near ultraviolet spectrum [7, 8]. Later on, cis-acrolein were detected in argon matrices [9, 10] and in the gas-phase Raman spectrum [11]. The first microwave detection of the cis form of acrolein in the gas phase was reported by Blom and Bauder [12]. They have reported the ground state rotational, quartic centrifugal distortion constants as well as dipole moment values. Blom et al. [13] also reported the complete substitution structures of both trans and cis conformers. The dipole moment values of the trans and cis-form of acrolein have been found to be μ=3.117±0.004 D [13] and μ=2.552±0,003 D [12] respectively. Winnewisser [14] have extended the analysis of the ground state of the trans-form of acrolein to the millimeter-wave region up to 180.0 GHz which has yielded a set of ground state rotational and centrifugal distortion constants. Analysis of the far-infrared spectrum of trans acrolein in the ν18 fundamental and (ν17+ν18)-ν18 hot bands were reported by McKellar et al. [15]. Very recently, 10 μm high-resolution rotational spectral analysis of the ν11, ν16, ν14 and ν16+ν18-ν18 bands of trans-acrolein were reported by Xu et al. [16]. In all the previous works trans-acrolein (propenal) was either procured commercially or prepared chemically.Production, identification, and spectroscopic characterization of new stable and transient molecules by applying a DC glow discharge through a low-pressure flow of gas or a mixture of gases inside an absorption cell have become a well-established area of research in the field of molecular spectroscopy [21]. Recently, Jaman et. al have reported analysis of the millimeter-wave rotational spectra of propyne (CH3CCH) [22] and propynal (HCCCOH) [23] produced by DC glow discharge technique and carried out detailed DFT calculations for both the molecules to evaluate the spectroscopic constants and molecular parameters and compared them with their respective experimental values. In the present communication, we report the analysis of the ground state (v=0) as well as several torsional excited states (v=1,2,3) rotational spectra of trans-propenal produced by a DC glow discharge through a low-pressure flow of allyl alcohol (CH2CHCH2OH) vapor in the frequency region 60.0–99.0 GHz. Asymmetric-top K-1 K+1-structures of different J+1←J transitions which falls under this frequency range have been observed and measured. The measured rotational transition frequencies along with the previously reported frequencies were fitted to standard asymmetric-top Hamiltonian to determine the rotational and centrifugal distortion (CD) constants for the ground as well as excited torsional states. A detailed quantum chemical calculation was also carried out to evaluate the spectroscopic constants, dipole moment, and the structural parameters of the trans conformer of propenal. Finally, the experimentally determined rotational and CD constants were compared with the best set of values obtained after a series of DFT calculations.


## 2. Experimental Details

The spectrometer used in the present work is basically a 50 kHz source-modulated system combined with a free space glass discharge cell of 1.5 m in length and 10 cm in diameter. The cell is fitted with two Teflon lenses at each end. A high voltage DC regulated power supply (6 kV, 1300 mA) procured from Glassman, Japan was used to apply a DC voltage through a flow of low pressure precursor gases. The cell is connected with a high vacuum pump at one end and to the sample holder section through a glass port on the other.Klystrons and Gunn diodes followed by frequency doubler (Millitech model MUD-15-H23F0 and MUD-10-LF000) have been used as radiation sources. Millimeter wave radiation was fed into the cell by a waveguide horn and Teflon lens. A similar horn and lens arrangement was used to focus the millimeter-wave power onto the detector after propagating through the cell. The output frequency of the millimeter wave radiation was frequency modulated by a bidirectional square-wave of 50 kHz [24] and the signal from the detector (Millitech model DBT-15-RP000 and DXP-10-RPFW0) was amplified by a 100 kHz tuned preamplifier and detected by a phase-sensitive lock in amplifier in the 2f mode. The output of the lock in amplifier was connected to an oscilloscope or a chart recorder for signal display. The spectrometer was calibrated by measuring standard OCS signals in the entire frequency range. After calibration, the uncertainty in frequency measurement has been estimated to be ±0.10 MHz. A block diagram of the spectrometer is shown in Figure 1. Details of the spectrometer used have been described elsewhere [25, 26].Figure 1
Block diagram of source-modulated millimeter wave spectrometer with DC discharge facility.Propenal (CH2CHCOH) was produced inside the absorption cell by applying a DC glow discharge through a low pressure (~5–10 mTorr) flow of allyl alcohol (CH2CHCH2OH) vapor. The discharge current was maintained at around 5 mA with an applied voltage of 1.0 kV. A mechanical on/off type discharge was found to be suitable to observe good signals of propenal. Signals could be observed at room temperature. However, a controlled flow of liquid nitrogen vapor through the cell helps in improving the signal intensity. The observed signals of propenal appeared as sharp lines immediately after the DC discharge was applied but started losing intensity with time.


## 3.  ComputationalMethod

Quantum chemical computations were performed using GAUSSIAN 09W package [27]. Density functional methods with various functionals were used to calculate the structural parameters, dipole moment, total energy (sum of electronic and zero point energy) as well as the rotational and centrifugal distortion constants of trans-propenal. The geometry optimization was carried out using different functionals like Becke 3-term correlation functional(B3LYP) with basis sets 6-31+g(d, p) to 6-311++g(d, 2p), Becke three-Parameter hybrid functional and Perdew/Wang 91 nonlocal correlation functional (B3PW91) method with different basis sets from 6-31 g to 6-311++g(d, 2p), modified Perdew-Wang one-parameter hybrid model taking basis sets from 6–31 g to 6-311++g(d, 2p) and Perdew, Burke, and Ernzerhof functional (PBEPBE) with the basis sets 6-311 g to 6-311++g(d, 2p). The frequency calculation along with its anharmonicity was done on optimized geometry. The objective of this DFT calculation is to compare the structural parameters and rotational constants of trans-propenal with the experimentally observed values in its ground state. The molecular drawing is done by using GAUSSVIEW5.0 [28].


## 4. Rotational Spectrum and Analysis

### 4.1. Ground State

The ground state rotational spectrum of thetrans conformer of propenal was predicted in the frequency range 60.0-99.0 GHz using the rotational and centrifugal distortion constants reported earlier [14]. J = 7←6 to J = 11←10 series of transitions along with their different K-1 K+1 components falls within this frequency range. Different components in each J+1←J series were measured. The observed lines were found very close to their predicted values. Finally, 224 a- and b-type R- and Q-branch transitions consisting of all previous microwave [1, 3], millimeter-wave [14], and present data were used to perform a kind of global fit to the semirigid rotor Watson’s S-reduction Hamiltonian (Ir-representation) [29] to determine a set of three rotational, five quartic, and two sextic centrifugal distortion constants. The shifts in frequency of the absorption lines from their rigid rotor positions due to centrifugal distortion effect were found to be less than that of propynal [23]. The observed and measured transition frequencies by us corresponding to J = 7←6 to 11←10 series are listed in Table 1. The ground state spectroscopic constants obtained for trans-propenal using the global fit are listed in Table 2. The small negative value of the inertia defect (Δ=-0.018 uÅ2) demonstrates that the equilibrium configuration of trans-propenal is planar. The agreement between the derived set of spectroscopic constants and those obtained earlier [1, 3, 14] with commercial samples indicates that the newly assigned transition frequencies of Table 1 definitely belong to trans-acrolein (trans-propenal), a discharge product of allyl alcohol vapor. Figure 2 shows the observed trace of the K-1=3 doublet of J = 9←8 transition immediately after the DC discharge was applied. The trace remained visible for a couple of minutes on the oscilloscope screen with gradually diminishing intensity.Table 1
Microwave and millimeter wave rotational transition frequencies oftrans-propenal (CH2CHCOH) in the ground and excited torsional states (in MHz.).
Transitions
Torsional levels
J
′
K
-
1
′
K
+
1
′
J
′
′
K
-
1
′
′
K
+
1
′
′
v
=
0
v
=
1
v
=
2
v
=
3
Obs. Freq.
Obs. − cal.
Obs. Freq.
Obs. − cal.
Obs. Freq.
Obs. − cal.
Obs. Freq.
Obs. − cal.
1
0
1
0
0
0
8902.19
−.01
2
0
2
1
0
1
17801.28
−.06
2
1
1
1
1
0
18221.12
−.07
18258.18
−.12
18289.86
−.01
3
0
3
2
0
2
26694.35
−.01
26824.88
.01
26895.18
.05
3
0
3
2
1
2
26765.43
−.09
27487.70
.06
3
1
2
2
1
1
27329.73
−.07
27385.58
.08
27432.89
.02
26322.50
−.07
3
1
3
2
1
2
26079.50
−.01
26165.84
−.05
26237.28
−.12
3
2
1
2
2
0
26718.70
−.16
3
2
2
2
2
1
26706.76
.04
4
0
4
3
0
3
35673.16
−.09
35752.72
.17
35846.64
.15
4
1
4
3
1
3
34768.96
−.11
7
0
7
6
0
6
62144.50a
.01
62312.22a
.01
62452.30a
−.11
7
1
7
6
1
6
60816.60a
.04
61018.40a
.02
7
2
6
6
2
5
62290.80a
−.06
62456.80a
−.03
62597.20a
.05
62760.10a
−.02
7
2
5
6
2
4
62460.30a
−.05
62624.20a
−.08
62762.20a
−.07
62922.30a
.03
7
3
4
6
3
3
62341.20a
−.01
62506.50a
−.08
62648.90a
.00
7
3
5
6
3
4
62339.40a
−.07
62504.80a
−.04
62647.20a
.03
7
4
3
6
4
2
62331.10a
.02
62496.50a
−.06
7
5
3
6
5
2
62493.40a
.03
8
0
8
7
0
7
70961.80a
.00
71154.30a
.10
71315.20a
.04
71506.60a
−.01
8
1
7
7
1
6
72820.00a
−.01
72969.40a
.03
73096.20a
.09
73243.30a
−.06
8
1
8
7
1
7
69489.80a
.06
69720.40a
−.12
69911.60a
.01
70139.80a
−.08
8
2
7
7
2
6
71179.00a
.07
71529.20a
.07
8
2
6
7
2
5
71432.70a
.12
71619.40a
.08
71776.20a
−.03
71958.30a
.04
8
3
6
7
3
5
71251.50a
.06
71440.40a
.05
8
3
5
7
3
4
71255.00a
.08
71443.90a
.07
71606.10a
−.17
8
4
4
7
4
3
71239.10a
.05
71428.20a
.09
8
5
4
7
5
3
71423.10a
.12
8
6
3
7
6
2
71421.10a
−.06
9
0
9
8
0
8
80371.70a
−.01
9
1
8
8
1
7
81901.20
.01
82069.50a
−.01
82212.30a
.06
9
2
8
8
2
7
80062.80a
.04
80276.30a
−.11
80457.00a
.08
80667.10a
.09
9
2
7
8
2
6
80424.00a
.07
80633.30a
.06
80808.70a
−.05
81012.40a
−.08
9
3
6
8
3
5
80172.10a
.05
80384.50a
.04
9
3
7
8
3
6
80165.70a
.03
80378.20a
.12
9
4
5
8
4
4
80148.40a
−.04
80361.10a
.04
9
5
5
8
5
4
80140.70a
.08
80353.40a
.06
9
6
4
8
6
3
80137.50a
−.01
80350.40a
.06
10
0
10
9
0
9
88523.30a
−.01
88765.90a
−.05
89211.90a
−.12
10
1
9
9
1
8
90974.40a
−.05
10
1
10
9
1
9
86818.50a
−.02
10
2
8
9
2
7
89436.20a
−.01
89667.83a
.03
89861.60a
−.02
10
2
9
9
2
8
88941.80a
−.01
89179.48a
.10
89380.10a
.07
89613.80a
−.02
10
3
7
9
3
6
89093.20a
−.01
89329.07a
−.01
10
3
8
9
3
7
89082.20a
−.08
10
4
6
9
4
5
89059.35a
−.09
89295.47a
−.09
10
5
5
9
5
4
89048.30a
−.01
10
6
4
9
6
3
89043.55a
−.04
10
7
3
9
7
2
89041.90a
−.02
10
9
1
9
9
0
89043.17a
−.02
11
0
11
10
0
10
97263.13a
−.01
97531.40a
−.03
97756.50a
−.12
98025.70a
.05
11
1
11
10
1
10
95473.05a
−.03
95791.10a
.01
96054.40a
.14
96369.80a
.04
11
2
9
10
2
8
98470.83a
−.01
98724.30a
−.11
98936.00a
.02
11
2
10
10
2
9
97815.59a
.03
98077.10a
.00
98555.60a
.08
11
3
8
10
3
7
98019.04a
−.02
98278.40a
.05
98500.60a
.03
98752.70a
−.03
11
3
9
10
3
8
98001.32a
.00
98260.60a
.01
98482.90a
−.06
98735.30a
−.01
11
4
8
10
4
7
98231.70a
−.08
11
5
7
10
5
6
97957.00a
.00
98216.90a
−.05
98453.50a
.04
11
6
5
10
6
4
97950.28a
.00
98456.20a
−.01
11
7
4
10
7
3
97947.54a
.01
11
8
3
10
8
2
97947.05a
.04
1
1
1
2
0
2
24892.58
.01
2
1
2
3
0
3
15585.86
.01
6
0
6
5
1
5
13444.27
.02
7
0
7
6
1
6
23450.46
.01
8
0
8
7
1
7
33595.68
−.01
aThis work, rest are from [1].Table 2
Ground state rotational and centrifugal distortion constants oftrans-propenal (CH2CHCOH).
Constants
Global fit using microwave and millimeter wave data
DFT calculation
A (MHz.)
47353.729± 0.009
47532.149
B (MHz.)
4659.4894± 0.0004
4635.391
C (MHz.)
4242.7034± 0.0004
4223.524
D
J (kHz)
1.031± 0.001
0.983
D
J
K (kHz)
−8.684± 0.006
−9.099
D
K (kHz)
361.949± 0.963
346.316
d
1 (kHz)
−0.1197± 0.0002
−0.119
d
2 (kHz)
−0.0069± 0.0001
−0.006
H
J
K (Hz)
0.014± 0.012
H
K
J (Hz)
−0.490± 0.021
σ
b
0.041
κ
c
−0.9806
Δ
d
−0.018
N
e
224
bStandard deviation of the overall fit.
cAsymmetry parameter.
dInertia defect Δ=Ic-Ib-Ia.
eNumber of transitions used in the fit.Figure 2
Observed trace of theK-1=3 doublet of J = 9←8 transition of trans-propenal produced by DC discharge.


### 4.2. Excited Torsional States

From an analysis of the ultraviolet [5] and far infrared spectrum [4] of acrolein vapor the first four excited torsional levels were found to lie around 157 cm−1 (v=1), 312 cm−1 (v=2), 468 cm−1 (v=3) and 623 cm−1 (v=4), respectively. Wagner et al. [3] have reported a few low J transitions of trans- acrolein in the 18.0–36.0 GHz for the first three (v=1, v=2 and v=3) excited torsional states and determined only the rotational constants B and C for each of these excited states. In this work, we have extended the analysis of rotational transitions in each of the above three excited states up to 99.0 GHz which has resulted in the determination of three rotational and two quartic centrifugal distortion constants for all the three torsional excited states. The new assigned transitions along with those reported earlier [3] are also shown in Table 1 along with the ground state transitions. The excited state data were also used to fit to the same semirigid rotor Watson’s S-reduction Hamiltonian (Ir-representation) [29]. Three rotational and two quartic (DJ and DJK) CD constants were used to fit the data. The contribution of other CD parameters was found to be negligible while fitting the excited state data. The derived spectroscopic constants and inertia defect values for the three torsional excited states are shown in Table 3. The more negative inertia defect values for successive torsional excited states indicate that the excited state lines arise from an out-of-plane vibration, in this case, COH group torsion about C–C single bond. The observed inertia defect values for the ground and torsional excited states of trans-propenal and some other related molecules are compared in Table 4.  The plots of inertia defect values with torsional quantum numbers for trans-propenal along with other molecules are shown in Figure 3 for comparison.Table 3
Excited-state spectroscopic constants oftrans-propenal (CH2CHCOH).
Constants
v
=
1
v
=
2
v
=
3
This work
Ref. [3]
This work
Ref. [3]
This work
Ref. [3]
A (MHz.)
45782.822± 3.231
44727.881± 3.873
43420.393± 5.888
B (MHz.)
4666.210± 0.004
4666.19± 02
4672.056± 0.005
4672.10± 0.02
4678.661± 0.006
4678.69± 0.02
C (MHz.)
4259.668± 0.005
4259.66± 0.02
4273.558± 0.006
4273.56± 0.02
4290.297± 0.007
4290.29± 0.02
D
J (kHz)
1.078± 0.012
1.280± 0.017
1.168± 0.026
D
J
K (kHz)
−8.735± 0.075
−46.831± 0.112
−28.714± 0.631
σ
f
0.077
0.085
0.075
κ
g
−0.9804
−0.9803
−0.9802
Δ
h
−0.702
−1.212
−1.861
N
i
43
28
20
fStandard deviation of the overall fit.
gAsymmetry parameter.
hInertia defect Δ=Ic-Ib-Ia.
iNumber of transitions used in the fit.Table 4
Comparison of the observed inertia defect (Δ/uÅ2) values for the ground and excited torsional state (v) of trans-propenal with some other molecules.
Molecules
Inertia defect (Δ/uÅ2) values
v
=
0
v
=
1
v
=
2
v
=
3
T
rans-propenalj
−0.018
−0.702
−1.212
−1.861
(CH2CHCHO)
o-cis 3-fluorobenzaldehydek
−0.078
−0.988
−1.876
−2.726
(C6H5FCOH)
Nitrobenzenel
−0.481
−1.863
−3.186
−4.470
(C6H5NO2)
Benzoyl fluoridem
−0.325
−1.528
−2.765
−3.963
(C6H5COF)
2-Fluorpstyrenen
−1.215
−2.689
−3.341
−4.380
(C6H4FC2H3)
jThis work, k[17], l[18], m[19], n[20].Figure 3
Plot of the variation of inertia defect values with torsional state quantum number fortrans-propenal and some other molecules.


### 4.3. Computational Results

Propenal is a slightly asymmetric prolate top molecule (κ=-0.9806). The optimization of geometry for the trans conformer of propenal was tested by employing various levels of theory and basis sets. However, the computed rotational and centrifugal distortion constants and the structural parameters obtained with model MPW1PW91 model with 6-31++g(d, 2p) basis set were found to be in good agreement with the observed values. Calculated values of ground state rotational constants of trans-propenal obtained with various models and basis sets are shown in Table 5. Results obtained with DFT MPW1PW91/6-31++g(d, 2p) have been compared with the corresponding experimental values in Table 2. For optimized geometry of trans-propenal the calculated energy is −5219.204 eV and the dipole moment is 3.481 D. The number and labeling of atoms in propenal molecule as shown in Figure 4. Bond lengths and angles have been computed using different models and basis sets and are shown in Tables 6 and 7, respectively.Table 5
Calculated ground state rotational constants oftrans-propenal (CH2CHCOH) with various models and basis sets.
Model
Basis set
A
0 (MHz)
B
0 (MHz)
C
0 (MHz)
DFT
B3LYP
6-31+g(d, p)
47421.368
4599.276
4189.265
B3LYP
6-31++g(d, 2p)
47462.833
4596.304
4190.461
B3LYP
6-311++g(d, 2p)
47713.427
4621.512
4213.560
B3PW91
6-31g
47562.896
4564.835
4164.742
B3PW91
6-31++g(d, 2p)
47445.030
4617.117
4207.640
B3PW91
6-311++g(d, 2p)
47640.454
4642.978
4230.737
MPW1PW91
6-31g
47680.556
4583.545
4181.241
MPW1PW91
6-31++g(d, 2p)
47532.149
4635.391
4223.524
MPW1PW91
6-311++g(d, 2p)
47719.894
4660.612
4246.017
PBEPBE
6-311g
46829.581
4528.525
4119.901
PBEPBE
6-311++g(d, 2p)
46855.368
4594.974
4175.330
PBEPBE
6-31++g(d, 2p)
46605.708
4567.810
4150.825
Expt.o
47353.729
4659.4894
4242.7034
oThis work.Table 6
Comparison of the molecular bond lengths, dipole moment, and total energy oftrans-propenal calculated by various methods and basis sets with the experimental values.
Models
Basis sets
Bond lengths between
Dipole Moment (D)
Energy (eV)
1C–2H
1C–3H
1C–4C
4C–5H
4C–6C
6C–7H
6C–8O
6-31+g(d, p)
1.088
1.085
1.340
1.087
1.474
1.112
1.218
3.515
−5220.511
B3LYP
6-31++g(d, 2p)
1.087
1.084
1.340
1.086
1.474
1.111
1.218
3.502
−5220.755
6-311++g(d, 2p)
1.084
1.081
1.335
1.083
1.474
1.109
1.211
3.464
−5221.979
6-31g
1.088
1.084
1.341
1.086
1.465
1.106
1.239
3.542
−5216.647
B3PW91
6-31++g(d, 2p)
1.085
1.082
1.334
1.084
1.471
1.111
1.208
3.491
−5218.334
6-311+g(d, 2p)
1.085
1.082
1.334
1.084
1.417
1.111
1.208
3.434
−5219.694
6-31g
1.086
1.083
1.339
1.085
1.463
1.104
1.236
3.564
−5217.218
MPW1PW91
6-31++g(d, 2p)
1.086
1.083
1.336
1.085
1.470
1.110
1.213
3.481
−5219.204
6-311++g(d, 2p)
1.084
1.081
1.332
1.083
1.469
1.109
1.205
3.443
−5220.238
6-311g
1.093
1.090
1.348
1.092
1.467
1.115
1.250
3.392
−5213.844
PBEPBE
6-31++g(d, 2p)
1.096
1.092
1.349
1.095
1.475
1.123
1.228
3.394
−5214.252
6-311++g(d, 2p)
1.093
1.089
1.344
1.092
1.474
1.121
1.221
3.367
−5215.531
Expt.p
1.089
1.081
1.341
1.084
1.468
1.113
1.215
3.117
pRef. [13].Table 7
Comparison of the molecular bond angles oftrans-propenal calculated by various methods and basis sets with the experimental values.
Models
Basis sets
Bond angles between
H2–C1–H3
H2–C1–C4
H3–C1–C4
C1–C4–H5
C1–C4–H6
H5–C4–C6
C4–C6–H7
C4–C6–O8
H7–C6–O8
6-31+g(d,p)
116.799
121.067
122.133
122.342
121.089
116.569
115.154
124.162
120.684
B3LYP
6-31++g(d,2p)
116.834
121.043
122.123
122.314
121.153
116.533
115.123
124.165
120.712
6-311++g(d,2p)
116.849
120.973
122.177
122.347
121.069
116.583
114.830
124.348
120.822
6-31g
116.488
121.219
122.22
122.091
121.314
116.596
115.381
123.994
120.626
B3PW91
6-31++g(d,2p)
116.699
121.104
122.196
122.363
120.926
116.711
115.126
124.248
120.625
6-311+g(d,2p)
116.922
120.852
122.226
122.414
120.821
116.766
114.689
124.414
120.897
6-31g
116.516
121.201
122.284
122.146
121.259
116.594
115.443
123.945
120.612
MPW1PW91
6-31++g(d,2p)
116.938
120.930
122.131
122.419
120.864
116.717
115.023
124.176
120.800
6-311++g(d,2p)
116.964
120.807
122.227
122.491
120.714
116.794
114.691
124.394
120.914
6-311g
116.573
121.132
122.295
122.136
121.724
116.139
115.404
123.451
120.604
PBEPBE
6-31++g(d,2p)
116.976
120.764
122.260
122.329
120.985
116.685
114.796
124.334
120.870
6-311++g(d,2p)
116.998
120.664
122.337
122.394
120.846
116.762
114.486
124.549
120.964
Expt.q
118.0
119.8
122.2
122.4
120.3
117.3
114.7
123.9
121.3
qRef. [13].Figure 4
Optimized geometry oftrans-propenal molecule and the numbering of atoms


## 4.1. Ground State

The ground state rotational spectrum of thetrans conformer of propenal was predicted in the frequency range 60.0-99.0 GHz using the rotational and centrifugal distortion constants reported earlier [14]. J = 7←6 to J = 11←10 series of transitions along with their different K-1 K+1 components falls within this frequency range. Different components in each J+1←J series were measured. The observed lines were found very close to their predicted values. Finally, 224 a- and b-type R- and Q-branch transitions consisting of all previous microwave [1, 3], millimeter-wave [14], and present data were used to perform a kind of global fit to the semirigid rotor Watson’s S-reduction Hamiltonian (Ir-representation) [29] to determine a set of three rotational, five quartic, and two sextic centrifugal distortion constants. The shifts in frequency of the absorption lines from their rigid rotor positions due to centrifugal distortion effect were found to be less than that of propynal [23]. The observed and measured transition frequencies by us corresponding to J = 7←6 to 11←10 series are listed in Table 1. The ground state spectroscopic constants obtained for trans-propenal using the global fit are listed in Table 2. The small negative value of the inertia defect (Δ=-0.018 uÅ2) demonstrates that the equilibrium configuration of trans-propenal is planar. The agreement between the derived set of spectroscopic constants and those obtained earlier [1, 3, 14] with commercial samples indicates that the newly assigned transition frequencies of Table 1 definitely belong to trans-acrolein (trans-propenal), a discharge product of allyl alcohol vapor. Figure 2 shows the observed trace of the K-1=3 doublet of J = 9←8 transition immediately after the DC discharge was applied. The trace remained visible for a couple of minutes on the oscilloscope screen with gradually diminishing intensity.Table 1
Microwave and millimeter wave rotational transition frequencies oftrans-propenal (CH2CHCOH) in the ground and excited torsional states (in MHz.).
Transitions
Torsional levels
J
′
K
-
1
′
K
+
1
′
J
′
′
K
-
1
′
′
K
+
1
′
′
v
=
0
v
=
1
v
=
2
v
=
3
Obs. Freq.
Obs. − cal.
Obs. Freq.
Obs. − cal.
Obs. Freq.
Obs. − cal.
Obs. Freq.
Obs. − cal.
1
0
1
0
0
0
8902.19
−.01
2
0
2
1
0
1
17801.28
−.06
2
1
1
1
1
0
18221.12
−.07
18258.18
−.12
18289.86
−.01
3
0
3
2
0
2
26694.35
−.01
26824.88
.01
26895.18
.05
3
0
3
2
1
2
26765.43
−.09
27487.70
.06
3
1
2
2
1
1
27329.73
−.07
27385.58
.08
27432.89
.02
26322.50
−.07
3
1
3
2
1
2
26079.50
−.01
26165.84
−.05
26237.28
−.12
3
2
1
2
2
0
26718.70
−.16
3
2
2
2
2
1
26706.76
.04
4
0
4
3
0
3
35673.16
−.09
35752.72
.17
35846.64
.15
4
1
4
3
1
3
34768.96
−.11
7
0
7
6
0
6
62144.50a
.01
62312.22a
.01
62452.30a
−.11
7
1
7
6
1
6
60816.60a
.04
61018.40a
.02
7
2
6
6
2
5
62290.80a
−.06
62456.80a
−.03
62597.20a
.05
62760.10a
−.02
7
2
5
6
2
4
62460.30a
−.05
62624.20a
−.08
62762.20a
−.07
62922.30a
.03
7
3
4
6
3
3
62341.20a
−.01
62506.50a
−.08
62648.90a
.00
7
3
5
6
3
4
62339.40a
−.07
62504.80a
−.04
62647.20a
.03
7
4
3
6
4
2
62331.10a
.02
62496.50a
−.06
7
5
3
6
5
2
62493.40a
.03
8
0
8
7
0
7
70961.80a
.00
71154.30a
.10
71315.20a
.04
71506.60a
−.01
8
1
7
7
1
6
72820.00a
−.01
72969.40a
.03
73096.20a
.09
73243.30a
−.06
8
1
8
7
1
7
69489.80a
.06
69720.40a
−.12
69911.60a
.01
70139.80a
−.08
8
2
7
7
2
6
71179.00a
.07
71529.20a
.07
8
2
6
7
2
5
71432.70a
.12
71619.40a
.08
71776.20a
−.03
71958.30a
.04
8
3
6
7
3
5
71251.50a
.06
71440.40a
.05
8
3
5
7
3
4
71255.00a
.08
71443.90a
.07
71606.10a
−.17
8
4
4
7
4
3
71239.10a
.05
71428.20a
.09
8
5
4
7
5
3
71423.10a
.12
8
6
3
7
6
2
71421.10a
−.06
9
0
9
8
0
8
80371.70a
−.01
9
1
8
8
1
7
81901.20
.01
82069.50a
−.01
82212.30a
.06
9
2
8
8
2
7
80062.80a
.04
80276.30a
−.11
80457.00a
.08
80667.10a
.09
9
2
7
8
2
6
80424.00a
.07
80633.30a
.06
80808.70a
−.05
81012.40a
−.08
9
3
6
8
3
5
80172.10a
.05
80384.50a
.04
9
3
7
8
3
6
80165.70a
.03
80378.20a
.12
9
4
5
8
4
4
80148.40a
−.04
80361.10a
.04
9
5
5
8
5
4
80140.70a
.08
80353.40a
.06
9
6
4
8
6
3
80137.50a
−.01
80350.40a
.06
10
0
10
9
0
9
88523.30a
−.01
88765.90a
−.05
89211.90a
−.12
10
1
9
9
1
8
90974.40a
−.05
10
1
10
9
1
9
86818.50a
−.02
10
2
8
9
2
7
89436.20a
−.01
89667.83a
.03
89861.60a
−.02
10
2
9
9
2
8
88941.80a
−.01
89179.48a
.10
89380.10a
.07
89613.80a
−.02
10
3
7
9
3
6
89093.20a
−.01
89329.07a
−.01
10
3
8
9
3
7
89082.20a
−.08
10
4
6
9
4
5
89059.35a
−.09
89295.47a
−.09
10
5
5
9
5
4
89048.30a
−.01
10
6
4
9
6
3
89043.55a
−.04
10
7
3
9
7
2
89041.90a
−.02
10
9
1
9
9
0
89043.17a
−.02
11
0
11
10
0
10
97263.13a
−.01
97531.40a
−.03
97756.50a
−.12
98025.70a
.05
11
1
11
10
1
10
95473.05a
−.03
95791.10a
.01
96054.40a
.14
96369.80a
.04
11
2
9
10
2
8
98470.83a
−.01
98724.30a
−.11
98936.00a
.02
11
2
10
10
2
9
97815.59a
.03
98077.10a
.00
98555.60a
.08
11
3
8
10
3
7
98019.04a
−.02
98278.40a
.05
98500.60a
.03
98752.70a
−.03
11
3
9
10
3
8
98001.32a
.00
98260.60a
.01
98482.90a
−.06
98735.30a
−.01
11
4
8
10
4
7
98231.70a
−.08
11
5
7
10
5
6
97957.00a
.00
98216.90a
−.05
98453.50a
.04
11
6
5
10
6
4
97950.28a
.00
98456.20a
−.01
11
7
4
10
7
3
97947.54a
.01
11
8
3
10
8
2
97947.05a
.04
1
1
1
2
0
2
24892.58
.01
2
1
2
3
0
3
15585.86
.01
6
0
6
5
1
5
13444.27
.02
7
0
7
6
1
6
23450.46
.01
8
0
8
7
1
7
33595.68
−.01
aThis work, rest are from [1].Table 2
Ground state rotational and centrifugal distortion constants oftrans-propenal (CH2CHCOH).
Constants
Global fit using microwave and millimeter wave data
DFT calculation
A (MHz.)
47353.729± 0.009
47532.149
B (MHz.)
4659.4894± 0.0004
4635.391
C (MHz.)
4242.7034± 0.0004
4223.524
D
J (kHz)
1.031± 0.001
0.983
D
J
K (kHz)
−8.684± 0.006
−9.099
D
K (kHz)
361.949± 0.963
346.316
d
1 (kHz)
−0.1197± 0.0002
−0.119
d
2 (kHz)
−0.0069± 0.0001
−0.006
H
J
K (Hz)
0.014± 0.012
H
K
J (Hz)
−0.490± 0.021
σ
b
0.041
κ
c
−0.9806
Δ
d
−0.018
N
e
224
bStandard deviation of the overall fit.
cAsymmetry parameter.
dInertia defect Δ=Ic-Ib-Ia.
eNumber of transitions used in the fit.Figure 2
Observed trace of theK-1=3 doublet of J = 9←8 transition of trans-propenal produced by DC discharge.


## 4.2. Excited Torsional States

From an analysis of the ultraviolet [5] and far infrared spectrum [4] of acrolein vapor the first four excited torsional levels were found to lie around 157 cm−1 (v=1), 312 cm−1 (v=2), 468 cm−1 (v=3) and 623 cm−1 (v=4), respectively. Wagner et al. [3] have reported a few low J transitions of trans- acrolein in the 18.0–36.0 GHz for the first three (v=1, v=2 and v=3) excited torsional states and determined only the rotational constants B and C for each of these excited states. In this work, we have extended the analysis of rotational transitions in each of the above three excited states up to 99.0 GHz which has resulted in the determination of three rotational and two quartic centrifugal distortion constants for all the three torsional excited states. The new assigned transitions along with those reported earlier [3] are also shown in Table 1 along with the ground state transitions. The excited state data were also used to fit to the same semirigid rotor Watson’s S-reduction Hamiltonian (Ir-representation) [29]. Three rotational and two quartic (DJ and DJK) CD constants were used to fit the data. The contribution of other CD parameters was found to be negligible while fitting the excited state data. The derived spectroscopic constants and inertia defect values for the three torsional excited states are shown in Table 3. The more negative inertia defect values for successive torsional excited states indicate that the excited state lines arise from an out-of-plane vibration, in this case, COH group torsion about C–C single bond. The observed inertia defect values for the ground and torsional excited states of trans-propenal and some other related molecules are compared in Table 4.  The plots of inertia defect values with torsional quantum numbers for trans-propenal along with other molecules are shown in Figure 3 for comparison.Table 3
Excited-state spectroscopic constants oftrans-propenal (CH2CHCOH).
Constants
v
=
1
v
=
2
v
=
3
This work
Ref. [3]
This work
Ref. [3]
This work
Ref. [3]
A (MHz.)
45782.822± 3.231
44727.881± 3.873
43420.393± 5.888
B (MHz.)
4666.210± 0.004
4666.19± 02
4672.056± 0.005
4672.10± 0.02
4678.661± 0.006
4678.69± 0.02
C (MHz.)
4259.668± 0.005
4259.66± 0.02
4273.558± 0.006
4273.56± 0.02
4290.297± 0.007
4290.29± 0.02
D
J (kHz)
1.078± 0.012
1.280± 0.017
1.168± 0.026
D
J
K (kHz)
−8.735± 0.075
−46.831± 0.112
−28.714± 0.631
σ
f
0.077
0.085
0.075
κ
g
−0.9804
−0.9803
−0.9802
Δ
h
−0.702
−1.212
−1.861
N
i
43
28
20
fStandard deviation of the overall fit.
gAsymmetry parameter.
hInertia defect Δ=Ic-Ib-Ia.
iNumber of transitions used in the fit.Table 4
Comparison of the observed inertia defect (Δ/uÅ2) values for the ground and excited torsional state (v) of trans-propenal with some other molecules.
Molecules
Inertia defect (Δ/uÅ2) values
v
=
0
v
=
1
v
=
2
v
=
3
T
rans-propenalj
−0.018
−0.702
−1.212
−1.861
(CH2CHCHO)
o-cis 3-fluorobenzaldehydek
−0.078
−0.988
−1.876
−2.726
(C6H5FCOH)
Nitrobenzenel
−0.481
−1.863
−3.186
−4.470
(C6H5NO2)
Benzoyl fluoridem
−0.325
−1.528
−2.765
−3.963
(C6H5COF)
2-Fluorpstyrenen
−1.215
−2.689
−3.341
−4.380
(C6H4FC2H3)
jThis work, k[17], l[18], m[19], n[20].Figure 3
Plot of the variation of inertia defect values with torsional state quantum number fortrans-propenal and some other molecules.


## 4.3. Computational Results

Propenal is a slightly asymmetric prolate top molecule (κ=-0.9806). The optimization of geometry for the trans conformer of propenal was tested by employing various levels of theory and basis sets. However, the computed rotational and centrifugal distortion constants and the structural parameters obtained with model MPW1PW91 model with 6-31++g(d, 2p) basis set were found to be in good agreement with the observed values. Calculated values of ground state rotational constants of trans-propenal obtained with various models and basis sets are shown in Table 5. Results obtained with DFT MPW1PW91/6-31++g(d, 2p) have been compared with the corresponding experimental values in Table 2. For optimized geometry of trans-propenal the calculated energy is −5219.204 eV and the dipole moment is 3.481 D. The number and labeling of atoms in propenal molecule as shown in Figure 4. Bond lengths and angles have been computed using different models and basis sets and are shown in Tables 6 and 7, respectively.Table 5
Calculated ground state rotational constants oftrans-propenal (CH2CHCOH) with various models and basis sets.
Model
Basis set
A
0 (MHz)
B
0 (MHz)
C
0 (MHz)
DFT
B3LYP
6-31+g(d, p)
47421.368
4599.276
4189.265
B3LYP
6-31++g(d, 2p)
47462.833
4596.304
4190.461
B3LYP
6-311++g(d, 2p)
47713.427
4621.512
4213.560
B3PW91
6-31g
47562.896
4564.835
4164.742
B3PW91
6-31++g(d, 2p)
47445.030
4617.117
4207.640
B3PW91
6-311++g(d, 2p)
47640.454
4642.978
4230.737
MPW1PW91
6-31g
47680.556
4583.545
4181.241
MPW1PW91
6-31++g(d, 2p)
47532.149
4635.391
4223.524
MPW1PW91
6-311++g(d, 2p)
47719.894
4660.612
4246.017
PBEPBE
6-311g
46829.581
4528.525
4119.901
PBEPBE
6-311++g(d, 2p)
46855.368
4594.974
4175.330
PBEPBE
6-31++g(d, 2p)
46605.708
4567.810
4150.825
Expt.o
47353.729
4659.4894
4242.7034
oThis work.Table 6
Comparison of the molecular bond lengths, dipole moment, and total energy oftrans-propenal calculated by various methods and basis sets with the experimental values.
Models
Basis sets
Bond lengths between
Dipole Moment (D)
Energy (eV)
1C–2H
1C–3H
1C–4C
4C–5H
4C–6C
6C–7H
6C–8O
6-31+g(d, p)
1.088
1.085
1.340
1.087
1.474
1.112
1.218
3.515
−5220.511
B3LYP
6-31++g(d, 2p)
1.087
1.084
1.340
1.086
1.474
1.111
1.218
3.502
−5220.755
6-311++g(d, 2p)
1.084
1.081
1.335
1.083
1.474
1.109
1.211
3.464
−5221.979
6-31g
1.088
1.084
1.341
1.086
1.465
1.106
1.239
3.542
−5216.647
B3PW91
6-31++g(d, 2p)
1.085
1.082
1.334
1.084
1.471
1.111
1.208
3.491
−5218.334
6-311+g(d, 2p)
1.085
1.082
1.334
1.084
1.417
1.111
1.208
3.434
−5219.694
6-31g
1.086
1.083
1.339
1.085
1.463
1.104
1.236
3.564
−5217.218
MPW1PW91
6-31++g(d, 2p)
1.086
1.083
1.336
1.085
1.470
1.110
1.213
3.481
−5219.204
6-311++g(d, 2p)
1.084
1.081
1.332
1.083
1.469
1.109
1.205
3.443
−5220.238
6-311g
1.093
1.090
1.348
1.092
1.467
1.115
1.250
3.392
−5213.844
PBEPBE
6-31++g(d, 2p)
1.096
1.092
1.349
1.095
1.475
1.123
1.228
3.394
−5214.252
6-311++g(d, 2p)
1.093
1.089
1.344
1.092
1.474
1.121
1.221
3.367
−5215.531
Expt.p
1.089
1.081
1.341
1.084
1.468
1.113
1.215
3.117
pRef. [13].Table 7
Comparison of the molecular bond angles oftrans-propenal calculated by various methods and basis sets with the experimental values.
Models
Basis sets
Bond angles between
H2–C1–H3
H2–C1–C4
H3–C1–C4
C1–C4–H5
C1–C4–H6
H5–C4–C6
C4–C6–H7
C4–C6–O8
H7–C6–O8
6-31+g(d,p)
116.799
121.067
122.133
122.342
121.089
116.569
115.154
124.162
120.684
B3LYP
6-31++g(d,2p)
116.834
121.043
122.123
122.314
121.153
116.533
115.123
124.165
120.712
6-311++g(d,2p)
116.849
120.973
122.177
122.347
121.069
116.583
114.830
124.348
120.822
6-31g
116.488
121.219
122.22
122.091
121.314
116.596
115.381
123.994
120.626
B3PW91
6-31++g(d,2p)
116.699
121.104
122.196
122.363
120.926
116.711
115.126
124.248
120.625
6-311+g(d,2p)
116.922
120.852
122.226
122.414
120.821
116.766
114.689
124.414
120.897
6-31g
116.516
121.201
122.284
122.146
121.259
116.594
115.443
123.945
120.612
MPW1PW91
6-31++g(d,2p)
116.938
120.930
122.131
122.419
120.864
116.717
115.023
124.176
120.800
6-311++g(d,2p)
116.964
120.807
122.227
122.491
120.714
116.794
114.691
124.394
120.914
6-311g
116.573
121.132
122.295
122.136
121.724
116.139
115.404
123.451
120.604
PBEPBE
6-31++g(d,2p)
116.976
120.764
122.260
122.329
120.985
116.685
114.796
124.334
120.870
6-311++g(d,2p)
116.998
120.664
122.337
122.394
120.846
116.762
114.486
124.549
120.964
Expt.q
118.0
119.8
122.2
122.4
120.3
117.3
114.7
123.9
121.3
qRef. [13].Figure 4
Optimized geometry oftrans-propenal molecule and the numbering of atoms


## 5. Conclusion

An efficient method of generatingtrans-propenal (trans-acrolein) in the gas phase by applying a DC glow discharge through a low pressure vapor of allyl alcohol inside the absorption cell has been presented. The gas phase rotational spectra of the trans conformer of propenal produced in this way has been recorded and analyzed in the frequency range 60.0–99.0 GHz for the ground as well as three torsional excited states (v= 1, 2 and 3). The asymmetric top K-1 K+1-components of different transitions having J values 6 to 10 have been measured. The observed transition frequencies along with the previously reported data [1, 3, 14] were fitted to a standard asymmetric-top Watson’s S-reduction Hamiltonian (Ir-representation) to determine ground state rotational and centrifugal distortion constants. Analysis of the rotational transitions for the three excited torsional states has been extended up to 99.0 GHz which has enabled us to determine the three rotational and two centrifugal distortion constants. The small negative value of the inertia defect (Δ=-0.018uÅ2) in the ground vibrational state (v=0) and the linear variation of the inertia defect values with torsional quantum number (v=1,2,3) demonstrate that the equilibrium configuration of trans-propenal is planar as noticed in case of 3-fluorobenzaldehyde, benzoyl fluoride, and nitrobenzene (Figure 4). The existence of a slightly bent or twisted–COH group would have resulted in a zig-zag behavior in the variation of inertia defect values with torsional quantum number as observed in the case of 2-fluorostyrene (Figure 4). To compare the experimental results with theory, DFT calculations were performed using various models and basis sets. However, it was found that MPW1PW91 model with 6-31++g (d, 2p) basis set produced the best values of rotational and quartric centrifugal distortion constants which are close to the experimental values.



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*Source: 363247-2012-08-22.xml*