Journal of Nuclear and Radiochemical Sciences, Vol. 23, pp. 20-25, 2023

ArticlesDevelopment of a gas chromatography-based method to derive halogen bonding energy ofAt compounds
Katsuyuki Tokoia, Atsushi Toyoshimab,*, Masashi Kanekoa, Atsushi Shinoharab,c,
Yoshitaka Kasamatsuaa

Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Institute for Radiation Sciences, Osaka University, Toyonaka, Osaka 560-0043, Japanc
Osaka Aoyama University, Minoh, Osaka 562-8580, Japanb

Received October 19, 2023; Accepted December 4, 2023; Published online January 10, 2024
Halogen bonding has attracted a great deal of attention in recent years due to its applications across various scientific fields. Halogen bonding of astatine (At) is theoretically predicted to be similar to that of iodine by taking into
account relativistic effects. In this study, we developed a new method based on gas chromatography and aided byquantum calculations to derive halogen bonding energy (HBE) of At compounds. A different methodology wasnecessary because the usual spectroscopic methods for HBE determination are not suitable for limited trace quantities of At compounds. We determined the adsorption enthalpies (ΔHs) of aromatic halogen compounds on thiourea by the developed gas chromatography-based method. The relationship between ΔH and calculated HBEs was
successfully obtained for halopentafluorobenzene and thiourea. We expect this relationship can be used to evaluate the HBE of astatopentafluorobenzene.
Keywords: halogen bonding, gas chromatography, quantum chemical calculation, Lewis base resin

1. Introduction
Halogen bonding has been widely utilized in molecular syntheses in various research endeavors including crystal engineering1, organic chemical reactions2, removal of halogenatedcompounds3, and design of pharmaceuticals 4. This peculiar
bonding of halogens is an intermolecular interaction between apositively polarized region on a halogen atom, which usuallyappears on the opposite side of its σ bond to such atoms as carbon, and a lone pair of a Lewis base (LB)5. The capability offluorine (F), chlorine (Cl), bromine (Br), and iodine (I) for halogen bonding increases with increasing atomic polarizationand decreasing atomic electronegativity6; the order is F < Cl <Br < I. However, there are few experimental reports that systematically investigate the halogen bonding properties. In particular, data on heavier halogens such as astatine (At) andtennessine (Ts) are very scarce. It is, therefore, very important
to study these heavier halogens for deeper understanding of
the halogen bonding.
In the periodic table of elements, At is placed under I. Basedon trends in homologous elements, At is expected to havestronger halogen bonding than lighter halogens have. On theother hand, At is expected to be strongly influenced by relativistic effects. Galland et al.7 theoretically investigated the interaction energy of the halogen bonding between At 2 andammonia and showed that the halogen bonding of At 2 is asstrong as that of I2 due to the spin-orbit interaction. Gallard et
al. reported the scalar relativity halogen bonding energy(HBE) of At 2 and ammonia to be –41 kJ/mol, whereas the
HBE value considering the spin-orbit interactions was –32 kJ/
mol. Thus, it is necessary to examine the influence of the spinorbit interaction in the halogen bonding of At.
Spectroscopic methods are often used to determine the HBE
which represents the strength of halogen bonding8. Because At*Corresponding author. E-mail: toyo@irs.osaka-u.ac.jp

has only short-lived radioisotopes with the longest being 210At(half-life of 8.1 h) and 211At (7.2 h), the available amount of At
for research studies is typically limited to less than 10 –12 mol.
This restriction prevents the use of any spectroscopic techniques to study the HBEs of At compounds. Guo et al.9 and
Liu et al.10 employed a solvent extraction method to determine
the formation constants of complexes forming the halogen
bonding between AtI and various LBs. From the comparisonof their obtained constants (logKBAtI) with literature values forthe corresponding formation constants of I 2 (logKBI2), GuO etal.9 found the relationship between the two constants. They
concluded that AtI has stronger halogen bonding than I 2 .
However, we consider it to remain uncertain whether the formation constants have a simple relationship with the HBE
because the inf luence of solvation has not been taken into
account.
In this paper, we developed a new derivation method of
HBEs of At compounds in trace quantities that is based on therelationship between calculated HBE values and adsorption
enthalpies (ΔHs) of lighter halogens which are experimentallyobtained by gas chromatography. The ΔH is a measure ofadsorption strength of compounds of interest onto a column
surface in a gas phase and can be applied to small quantities ofcompounds11. The ΔH involves van der Waals forces, dipoles,
and hydrogen bonding12. Lee et al.13 have explored the relation
between the ΔHs of alkylamines (LB) on an acidic zeolite surface in which hydrogen bonding is involved and the proton
affinities of the alkylamines that are an indicator of the abilityof an LB to accept protons. They found a linear relationshipbetween the ΔHs and the proton affinities. An analogous relationship can readily be thought to exist between halogen bonding and hydrogen bonding. Both interactions are based on anelectron donor/electron acceptor relationship involving a halogen-like atom and an electron-dense counterpart 5. Therefore,
because proton affinity is similar to HBE, ΔH is expected to© 2023 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 01/10/2024

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J. Nucl. Radiochem. Sci., Vol. 23, 2023

have a linear relationship with HBE.
We describe our investigation of the relationship between
calculated HBE and ΔH of lighter halogens for the purpose ofdeveloping a derivation method for HBE of At compounds byextrapolation. First, gas chromatography experiments using
halogen compounds on a thiourea resin were carried out todetermine their ΔHs. Thiourea was selected because it isexpected to have strong halogen bonding10. HBEs of the corresponding complexes were then obtained using quantum calculations. To ensure the calculation accuracy, we compared theredshift of the C-X stretching vibration measured by Raman
spectroscopy14 with the calculated results of the C-X stretchingvibration. Finally, the relationship between the measured ΔHs
and calculated HBEs was evaluated.
2. Experimental
2.1 Materials. Halopentafluorobenzene (C6F5X, X = Cl, Br,
and I) and halobenzene (C6H5X) were purchased from Tokyo
Chemical Industry Co. Hexane was purchased from Fujifilm
Wako Pure Chemicals Co. All reagents are commerciallyavailable and they were used without additional purification.
Thiourea supported on silica gel (Si-Thiourea resin, R69530B)
was purchased from SiliCycle Inc. The Si-thiourea resin has aloading amount of 1.20 mmol/g and can be used at temperatures to 150°C.
2.2 Gas chromatography using LB resin. As studied samples, C6F5X and C6H5X were used because they are expected to
exhibit strong and weak retentions on the LB Si-thiourea resin,
respectively, according to their halogen bonding. A gas chromatograph (Shimadzu GC-2014) equipped with a thermal conductivity detector was used for the experiments. Teflon tubing(800 mm long, 3 mm inner diameter, and 5 mm outer diameter) was utilized as a column which was filled with the LB
Si-Thiourea resin to a packed length of 500 mm. The sample
inlet temperature was set to 160°C. The column temperature
was kept at 140, 145, and 150°C during the experiments.
Helium was chosen as a carrier gas with a f low rate of
10 mL/min. A syringe was employed to inject 4 – 12 μL of asample. The acquired chromatograms were fitted with theGaussMod function to determine the retention time15. The
retention times here were expressed as follows
t′r = tr − tm ,(1)

where tr’ is the adjusted retention time, tr is the observed retention time, and tm is the time through the empty column.
2.3 Raman spectroscopy. For the Raman spectroscopy
measurements, a mixed sample of C6F5I and Si-Thiourea resin
and a C6F5I sample were prepared. In a 1.5-mL polypropylene
tube, 0.23 mmol of C6F5I and 98 mg of the Si-thiourea resin
were mixed and kept at room temperature for 11 h. For the
C6F5I sample, 200 μL of C6F5I and 1600 μL of hexane weremixed in a quartz measurement vial; the concentration of
C6F5I was 0.84 M. Hexane was used as a solvent because it candissolve C6F5I and has no peak in the energy range of interestin a Raman spectrum. Raman spectra were measured using a
micro-Raman spectrometer (Jasco Co. NRS3100) with laser
wavelength of 750 nm at room temperature. Each spectral
point was measured 5 times with an exposure time of 60 s
each.
2.4 Quantum chemical calculation. All quantum calculations were carried out using Gaussian 16 software16. Initialgeometries of the molecules (C6H5X, C6F5X, and thiourea) andthe donor-acceptor complexes with a ratio of 1:1 (C 6 H 5 Xthiourea and C 6 F5 X-thiourea complexes) were constructedusing Gaussview 6.1 . Structures of all the compounds were
optimized using the density functional theory calculation withthe ωB97XD functional18, followed by single-point energy calculations using second-order Møller-Presset perturbation theory (MP2). Aug-cc-pVDZ basis sets19,20,21 were employed for
all atoms except iodine, and the DGDZVP basis set 22,23 was
assigned to iodine. The basis sets were employed for both thegeometry optimization and single-point energy calculations.
All the self-consistent field calculations were performed undertight convergence criteria, in which the tolerance of energy
was set to 10−6 Hartree during the iterations. The HBE for eachcomplex was calculated by using the total energies obtained in
the single-point calculations, as shown in equation (2)17HBE = {E tot(complex) + E tot
BSSE (complex)} −{E tot(donor) + E tot(acceptor)}(2)

Here Etot denotes the total energy and EtotBSSE is the correction
term for the basis set superposition error based on the counterpoise method24.
In the benchmark calculation, the computational conditions(e.g., basis set, calculation method) were optimized based on
experimental HBE values for the CF3Cl-NH3 system8. Next, toevaluate the validity of the calculation conditions in the studied complex of C 6 F5 I-thiourea, the peak positions of theRaman shift of the C-I bonding were calculated under thedetermined conditions and compared with experimental values. In the vibrational frequency calculation to obtain the
Raman spectra, the solvent effect was taken into consideration
using the polarizable continuum model (PCM) with the dielectric constant of the solvent25.
3. Results and Discussion
3.1 Gas chromatography using the LB Si-thiourea resin.
Figure 1 shows a chromatogram of each compound obtainedwith a column temperature of 145°C. The obtained chromatograms were fitted as the red curves in Figure 1. Retention time
was evaluated from the fitted peak. Figures 2 and 3 show theretention times of C6F5X and C6H5X, respectively, at various
column temperatures. The decrease of retention time as afunction of column temperature was fitted with equation (3)26,
t′r =

A×B×L
–ΔH
exp
,
F
RT

(3)

where A is the interstitial area of the column (m²), B is a constant (showing entropy change in adsorption), L is the length ofthe packing material (m), F is the flow rate (mL/min), ΔH isthe adsorption enthalpy (kJ/mol), T is the column temperature(K), and R is the gas constant (J/mol K). Table 1 summarizes
the calculated ΔH values of halogen compounds obtainedusing equation (3).
We observed an increasing variation of ΔHs for heavier halogens for C6F5X, which is consistent with the general trend of
HBEs. On the other hand, C6H5X exhibited nearly identical
ΔH values. Hogan et al.27 calculated energies of hydrogen andhalogen bonding between water and C6H5X and showed thathydrogen bonding is dominant over halogen bonding. Thissuggests a significant contribution of hydrogen bonding toadsorption behavior of C6H5X in the present study. Thus, weconsidered the observed nearly identical ΔHs of C6H5X originate from hydrogen bonding between C6H 5X and silica gel.
Since the C-F---H-O hydrogen bonding is weak, the involvement of hydrogen bonding in the ΔH of C6F5X is expected tobe small28.
3.2 Raman spectroscopy. Table 2 lists the Raman spectra
peaks of C6F5I and of C6F5I adsorbed on Si-thiourea resin. A
distinct peak at 207 cm -1 attributed to the C-I bond was

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J. Nucl. Radiochem. Sci., Vol. 23, 2023

Development of a gas chromatography-based method to derive halogen

Figure 1. Chromatograms of each sample obtained at a column temperature of 145°C. (a) C6H5Cl, (b) C6H5Br, (c) C6H5I, (d) C6F5Cl, (e) C6F5Br,
and (f) C6F5I. Red lines are fitted curves.

Cl
Br
I

100

Retention time / min

Retention time / min

100

50

Cl
Br
I

50

0

0
410

415

420

410

425

415

420

425

Temperature / K

Temperature / K
Figure 2. Variations of retention times of C6F5X (X = Cl, Br, and I)
as a function of column temperature. Each solid line represents thefitted curve.

Figure 3. Variations of retention times of C6H5X (X = Cl, Br, and I)
as a function of column temperature. Each solid line represents thefitted curve.

TABLE 1: Calculated ΔH values for halogen compounds
C6F5X and C6H5X.

TABLE 2: Experiment and quantum chemical calculation
values of Raman peaks.

Element

ΔH, kJ mol−1
C6F5X

C6 H 5 X

Cl−47.0 ± 1.7−56.0 ± 3.2

Br−55.0 ± 7.5−54.1 ± 1.4

I−67.5 ± 2.3−58.3 ± 7.9

Samplesa

C-I Raman shift, cm−1
ExperimentCalculation

C6F5I

207

208

C6F5I + thiourea resin

197

198a

Calculations were performed on C6F5I-thiourea complex.

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J. Nucl. Radiochem. Sci., Vol. 23, 2023

Figure 4. Structures of the halogen bonded C6F5I-thiourea complex obtainedin the quantum chemical calculation.

TABLE 3: Optimized structures of C6F5X-thiourea and C6H5X-thiourea complexes.
Element

C-X-S angle, °C6F5X-thiourea

C6H5X-thiourea

C6F5X-thiourea

C6H5X-thiourea

Cl

3.538

–

a

159.712

–a

Br

3.346

3.623

168.099167.121

I

3.346

3.616

172.778

171.062

The geometry optimization of C6H5Cl-thiourea failed to converge.

observed for the C6F5I sample. For the C6F5I sample adsorbedon the resin, the C-I peak was found at 197 cm–1, indicating theredshift by the halogen bonding. Bramlett et al.29 reported theC-I peak of C6 F5I in dichloromethane was at 204 cm –1. The
slight discrepancy between the present value and the literature
value29 would be due to the different solvents of the measured
samples30.
3.3 Quantum chemical calculations. In determining thebasis set conditions, benchmark calculations were performed
using the HBE of CF3Cl···NH3 measured in reference 8 whichis –11.0 kJ/mol. Using aug-cc-pVDZ as the basis set at theMP2 level yielded HBE of –10.0 kJ/mol that well reproduced
the reported experimental value8. We further verified that aclose value was obtained with a structure optimized using
ωB97XD.
For the structure optimization and vibrational analysis withthe ωB97XD, aug-cc-pVDZ was used for non-iodine atoms,
and DGDZVP was used for iodine atoms. The results are listed
in Table 2. Under these calculation conditions, the peak positions of C-I, with and without thiourea, were 208 and 198 cm–1,
respectively. The experimental and calculated Raman spectra
for this system were in a good agreement with each other, indicating that the present calculation conditions were reliably
applied to the HBE calculations.
Figure 4 and Tables 3 and 4 give the results of structural
optimization and HBE calculations conducted with the optimized conditions of the calculation method and basis sets. For
C 6 H 5Cl, HBE values were unavailable since the structural
optimization did not converge, possibly due to the weakernature of the halogen bonding ability of Cl compared to that of
the other halogens.
Similar to previous findings 31, the increase in HBE wasobserved as the atomic number of the halogen increased.
C 6 F5 X had a larger HBE due to the electron-withdrawing
effect of the aromatic ring fluorine31.

TABLE 4: Results of HBE calculations based on quantum
chemical calculations.
Element

HBE, kJ mol−1
C6F5X-thiourea

C6H5X-thiourea

Cl

−15.9

–

Br

−20.1

−14.3

I

−24.8

−17.0

100

■C6F5X▲C6H5X80-ΔH / kJ mol-1a

X-S distance, Å

I
I

60

Br

40

Br
Cl

20
0
0

10

20

30

40

50

-1

-HBE / kJ mol

Figure 5. Relationship between ΔHs and HBEs of C6F5X and C6H5X(X = Cl, Br, and I). The red line is the fitted straight line for the
C6F5X data.

Development of a gas chromatography-based method to derive halogen

3.4 Relationship between ΔH and calculated HBE.
Figure 5 shows the relationship between ΔHs and calculated
HBEs of our studied compounds. For C6F5X, ΔHs and calculated HBEs demonstrated a good linear relationship with thePearson correlation coefficient of 0.9995. The fitted line isexpressed as
ΔH = (2.3 ± 0.1) × HBE + (10.4 ± 1.4) kJ/mol.

(4)

As observed in the earlier study13, a linear relationship wasobserved in the calculated HBE vs. ΔH in the present reaction
system. The observed HBE dependence of ΔH suggests that
halogen bonding cont ributes to the adsor ption on the
Si-thiourea resin. There are halogen bonding modes involving
multiple LBs, which are known to increase calculated HBE
compared to single halogen bonding32. Thus, the slope of equation (4) is considered to be greater than 1 because of theinvolvement of multiple halogen bonding interactions. In thenext step, the investigation of multiple halogen bonding models with quantum chemical calculations should be involved.
The value of the intercept is expected to represent the strengthof interactions such as van der Waals forces without halogen
bonding in ΔH. We, therefore, concluded that the HBE of
C6F5X can be inferred from the ΔH value determined by gas
chromatography, i.e., the equation of the relationship between
ΔH and the calculated HBE in homologous elements may be
used to evaluate the HBE of C6F5At.
4. Conclusion
The ΔH of C6F5X was determined through gas chromatography experiments utilizing a Si-thiourea resin as a Lewisbase. The relationship between ΔH and HBE was successfully
obtained. We expect the HBE value of astatopentafluorobenzene can be derived from its ΔH determined with gas chromatography using the developed methodology. By comparing the
HBE value with that from relativistic quantum calculations, it
may become possible to explore the influence of relativistic
effects on the halogen bonding of At compounds.
Acknowledgements
This work was the result of using research equipment sharedin the MEXT Project for promoting public utilization ofadvanced research infrastructure (Program for supportingc o n s t r u c t i o n o f c o r e f a c i l i t i e s) G r a n t N u m b e r
JPMXS0441200023. One author (KT) acknowledges financial
suppor t from the Multidisciplinar y Ph.D. Program for
Pioneering Quantum Beam Application by Osaka University
and the SPRING program by Japan Science and Technology
Agency.
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