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Vibrational spectrum, ab initio calculation, conformational equilibria and torsional modes of 1,3-dibromopropane
Spectrochimica Acta Part A 61 (2005) 1547–1557
Vibrational spectrum, ab initio calculation, conformational equilibria and torsional modes of 1,3-dibromopropane Matthew S. Nalewanski a, 1 , Yann P. Tambouret a, 1 , Scott T. Lentini a, 1 , Howard D. Stidham a, ∗ , Gamil A. Guirgis b b
a Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA Chemistry and Biochemistry Department, College of Charleston, Charleston, SC, USA
Received 26 October 2004; accepted 10 November 2004 Dedicated to Professor James R. Durig on the occasion of his 70th birthday as an excellent spectroscopist and great friend.
Abstract The infrared and Raman spectrum of 1,3-dibromopropane is reported in the crystalline, liquid and gaseous states. These measurements are compared to the results of ab initio calculations carried out using the 6-31+g* Gaussian basis set for a restricted Hartree–Fock computation. The calculation was repeated using second order Moeller–Ploesset perturbation theory to accommodate electron correlation using the 6-31g* basis set. The three most stable conformers are GG (C2 ), AG (C1 ) and AA (C2v ), where A and G stand for anti and gauche orientations of the bromomethyl group relative to the plane of the carbon atoms. The point group symmetry of each structure is given in parentheses. The fourth conformer, G G (Cs ) is of such high energy that it is not observed experimentally in isotropic media in either the infrared or Raman spectrum. In the crystalline state, comparison of the infrared and Raman spectrum with that calculated for the C2 conformer shows that only the GG (C2 ) conformer survives, and the doublet structure of many of the bands in the spectrum indicates at least two molecules per unit cell. The ab initio calculations predict and the temperature dependence of the Raman spectrum of the liquid confirms that the stability order is C2 < C1 < C2v Cs . These data show that in the liquid the C1 conformer lies 220 ± 30 cm−1 above the lowest energy C2 conformer, while the C2v is 435 ± 60 cm−1 above it. A nearly complete assignment of fundamentals is possible for the C2 conformer, and a number of bands can be confidently identified for the C1 . However, only two weak Raman bands with very weak infrared counterparts could be confidently assigned to the C2v conformer. A complete scaled normal coordinate calculation was conducted. The spectrum calculated from the results agrees very well with the observed. The average error for the C2 conformer could be reduced to as little as 0.6% (6.8 cm−1 ) by appropriate scaling. © 2004 Published by Elsevier B.V. Keywords: Vibrational spectrum; Ab initio calculations; Conformational equilibria 1,3-dibromopropane
1. Introduction The 1,3-dihalopropanes have been shown to have four spectroscopically distinct conformers [1–6]. If G and A refer to the gauche and anti orientations of the halomethyl groups with respect to the plane of the carbon atoms, these are GG (C2 ), AG (C1 ), AA (C2v ) and GG (Cs ), where the molecular point group of the isolated structure is indicated ∗ 1
Corresponding author. Tel.: +1 413 545 0048; fax: +1 413 545 4490. E-mail address: [email protected] (H.D. Stidham). Undergraduate participants.
1386-1425/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.saa.2004.11.060
in parentheses. These structures have statistical weights of 2, 4, 1 and 2, respectively. From electron diffraction measurements made on the gas at 35 ◦ C, the relative stabilities of 1,3-dibromopropane have been reported as 67% GG, 30% AG and 3% AA [6]. Since both bromine nuclei are on the same side of the plane of the carbon atoms in the GG conformer, this conformer has a much higher energy than the others and is not observed in the gas or liquid near room temperature [1,6]. A schematic drawing of the four conformers is shown in Fig. 1. In early work, Cochran et al. [7] reported spectroscopic work on 1,3-dibromopropane, but in later work Crowder
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Fig. 1. Conformers of 1,3-dibromopropane.
[8] suggested that several fundamentals were misassigned. Thorbjornsrud et al. [1] then published an extensive study of the infrared and Raman spectra of several dihalo substituted propanes and assigned many of the fundamentals of the more stable conformers. The present work was undertaken as a logical extension of our earlier studies [2,4,5] in the expectation that a much more definitive assignment of the observed fundamentals could be made on the basis of a comparison of the infrared and Raman spectra in various states of aggregation with the results of ab initio and scaled normal coordinate calculation of the frequencies. Owing to limitations on the available computers, the calculations could not be performed at quite as high a level of the theory as was possible with the earlier studies. Normal coordinate calculations were done only with the C2 , C1 and C2v conformers using the results of rhf/6-31+g* and mp2 = full/6-31g* Gaussian98 [9] calculations. The scaled calculations, however, provide eerily accurate calculated spectra differing in the liquid state only slightly from the observed in only a few frequency or intensity details. This suggests that the composition of the liquid state at room temperature is nearly 56% C2 , 39% C1 and 5% C2v , in excellent agreement with the electron diffraction results of Farup and Stolevik [6], obtained for the gas at a slightly higher temperature.
2. Experimental 1,3-Dibromopropane (Sigma–Aldrich) was purified by trap to trap distillation on an all PyrexTM glass vacuum line, but was used otherwise as received. The purity stated by the supplier was 99%. Infrared spectra were obtained with a Mattson Cygnus 100 Fourier transform infrared interferometer operated at resolutions of 1 cm−1 for condensed phase and 0.5 cm−1 for the gas. Raman spectra were obtained with a Spex model II double monochromator equipped with 1200 grooves per mm gratings and a thermoelectrically cooled Hammamatsu R943-02 photomultiplier worked with locally designed photon counting electronics. Spectrometer advance and data collection
was done under computer control, using a National Instruments PCI-6602 dual counter–timer interface board operated with Labview VIs of local design to count photomultiplier pulses and drive the stepper motor that advances the spectrometer wavenumber setting. Raman spectra were excited by a Spectra-Physics Model 177-G42 air cooled Ar+ ion laser typically operated to deliver 100 mW of 514.5 nm continuous radiation at the laser head. Polarization measurements were made with a Polaroid analyzer set before a quartz polarization scrambler plate placed before the entrance slit of the monochromator. The Raman spectrum of the liquid was obtained by transillumination of PyrexTM glass capillaries filled and sealed under running vacuum while the sample was frozen in liquid nitrogen. Infrared and Raman spectra of the solids were obtained with evacuated all glass cold cells of local construction. The infrared cold cell was equipped with KBr windows and the sample was deposited under running vacuum onto a freshly polished CsI plate chilled to near liquid nitrogen temperatures. The Raman cold cell was equipped with a flat optical glass window. The sample was deposited on a brass block blackened to suppress laser reflections, by flaming an aqueous suspension of amorphous graphite onto the block, giving a surface very different from the CsI plate used in the infrared cold cell.
3. Results and discussion The results of the structural optimization for two of the ab initio calculations done are given in Tables 1 and 2. It is noteworthy that the restricted Hartree–Fock energy calculations using the 6-31+g* basis set reverses the order of stabilities for the C1 and C2v conformers. The Moeller–Ploesset full second order electron correlation (mp2) for the 6-31g* basis set retains the order, but predicts much too large energy differences. This was further investigated, and it was found that an inversion in the order was predicted by no other basis set. When the basis was specified as 6-31+g(d,p), the same inversion and essentially the same relative energy was found, and when a density functional calculation B3LYP for 6-31+g(d) basis was conducted, the inversion occurred again. The other basis sets tried are shown in Table 3, together with the relative energies of the conformers expressed in cm−1 units these produced. None of these inverted the relative energies, but also none calculated the experimentally observed well. To some extent, the differences between the highest level basis set results and experiment may be attributed to dipole–dipole interaction, which will be much more prevalent in the liquid than in the gas to which the ab initio results are appropriate. However, the calculated dipole moments of the various conformers indicate that only the Cs conformer ought to be appreciably stabilized by the interaction, since it has a calculated dipole moment of about 4 Debye. The three observable conformers have dipole moments of only 2–2.5 Debye (see for details Tables 1 and 2). There was nothing in these
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Table 1 Computational results for 1,3-dibromopropane using the rhf/6-31+g* basis Parameter
GG (C2 )
AG (C1 )
AA (C2v )
r(C1 –C2 ) r(C1 –C3 ) r(C1 –H4 ) r(C1 –H5 ) r(C2 –Br6 ) r(C2 –H7 ) r(C2 –H8 ) r(C3 –Br9 ) r(C3 –H10 ) r(C3 –H11 ) ∠(C2 –C1 –C3 ) ∠(C2 –C1 –H4 ) ∠(C3 –C1 –H4 ) ∠(C2 –C1 –H5 ) ∠(C3 –C1 –H5 ) ∠(H4 –C1 –H5 ) ∠(C1 –C2 –Br6 ) ∠(C1 –C2 –H7 ) ∠(Br6 –C2 –H7 ) ∠(C1 –C2 –H8 ) ∠(Br6 –C2 –H8 ) ∠(H7 –C2 –H8 ) ∠(C1 –C3 –Br9 ) ∠(C1 –C3 –H10 ) ∠(Br9 –C3 –H10 ) ∠(C1 –C3 –H11 ) ∠(Br9 –C3 –H11 ) ∠(H10 –C3 –H11 ) τ(C3 –C1 –C2 –Br6 ) τ(Br9 –C3 –C1 –C2 ) A (cm−1 ) B (cm−1 ) C (cm−1 ) |µa | (D) |µb | (D) |µc | (D) |µt | (D) (E+5256)(hartree) Raman relative energy, liquid (cm−1 ) rhf/6-31+g* relative energy, gas (cm−1 )
1.5182 1.5182 1.087 1.087 1.9594 1.078 1.077 1.9594 1.078 1.077 116.01 109.39 107.23 107.23 109.39 107.30 112.39 110.81 105.31 112.64 105.57 109.74 112.39 110.81 105.31 112.64 105.57 109.74 59.30 59.30 0.1479 0.0221 0.0207 0 0 2.9059 2.9059 0.9292469 0 0
1.5182 1.5205 1.0828 1.0869 1.9494 1.0792 1.0771 1.9605 1.0794 1.0781 112.30 110.47 110.15 109.22 107.18 107.34 111.28 111.98 105.58 112.17 105.83 109.62 112.70 112.75 105.81 110.97 104.87 109.32 −177.51 −69.68 0.2570 0.01526 0.01471 −0.2735 1.7325 1.7106 2.4501 0.9270859 250 ± 30 474
1.5205 1.5205 1.0831 1.0831 1.9468 1.0793 1.0793 1.9468 1.0793 1.0793 108.57 110.16 110.16 110.16 110.16 107.63 112.37 112.07 105.35 112.07 105.35 109.21 112.37 112.07 105.35 112.07 105.35 109.21 180.0 180.0 0.4592 0.01236 0.01213 0 0 2.1279 2.1279 0.9281609 435 ± 70 238
Bond distances are in Angstroms, bond angles are in degrees and dipole moments are in Debyes.
calculations that could identify the source of the inversions found, and the origin remains undetermined. The observed infrared and Raman spectrum in the gaseous, liquid and crystalline solid state are shown in Figs. 2–5. Experimentally observed frequencies are collected in Table 4, together with the assignments of fundamentals for the observed bands. The frequencies and infrared and Raman intensities were calculated using the basis sets of Tables 1 and 2. The force field in Cartesian coordinates was used to perform a normal coordinate calculation, using the internal coordinates quoted in Table 5, some of which are displayed in Fig. 6. When the frequencies calculated by the Gaussian suite of programs were reproduced, scaling was initiated. The scaling factors used for the final calculation were determined by matching experimental frequencies assigned to the C2 conformer as closely as possible. The scaling factors determined for the C2
Fig. 2. Infrared spectrum of 1,3-dibromopropane from 400 to 1500 cm−1 : (A) gas; (B) liquid; (C) annealed crystal near 77 K.
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Table 2 Computational results for 1,3-dibromopropane using the mp2 = full/6-31G* basis Parameter
GG (C2 )
r(C1 –C2 ) r(C1 –C3 ) r(C1 –H4 ) r(C1 –H5 ) r(C2 –Br6 ) r(C2 –H7 ) r(C2 –H8 ) r(C3 –Br9 ) r(C3 –H10 ) r(C3 –H11 ) ∠(C2 –C1 –C3 ) ∠(C2 –C1 –H4 ) ∠(C3 –C1 –H4 ) ∠(C2 –C1 –H5 ) ∠(C3 –C1 –H5 ) ∠(H4 –C1 –H5 ) ∠(C1 –C2 –Br6 ) ∠(C1 –C2 –H7 ) ∠(Br6 –C2 –H7 ) ∠(C1 –C2 –H8 ) ∠(Br6 –C2 –H8 ) ∠(H7 –C2 –H8 ) ∠(C1 –C3 –Br9 ) ∠(C1 –C3 –H10 ) ∠(Br9 –C3 –H10 ) ∠(C1 –C3 –H11 ) ∠(Br9 –C3 –H11 ) ∠(H10 –C3 –H11 ) τ(C3 –C1 –C2 –Br6 ) τ(Br9 –C3 –C1 –C2 ) A (cm−1 ) B (cm−1 ) C (cm−1 ) |µa |(D) |µb | (D) |µc | (D) |µt | (D) –(E + 5257)(hartree) Raman relative energy, liquid (cm−1 ) mp2/6-31g* relative energy, gas (cm−1 )
1.5155 1.5155 1.0966 1.0966 1.9611 1.0894 1.089 1.9611 1.0894 1.089 114.2052 109.4089 107.9747 107.9747 109.4089 107.6855 111.1308 111.7576 105.7923 111.783 105.9317 110.124 111.1308 111.7576 105.7923 111.783 105.9317 110.124 61.9951 61.9951 0.1555 0.0217 0.0205 0 0 2.6122 2.6122 0.5732771 0 0
AG (C1 )
AA (C2v )
1.516 1.5176 1.0935 1.0965 1.9566 1.0908 1.0883 1.9617 1.0907 1.0898 112.2494 109.7385 119.9874 109.3704 107.9299 107.4297 110.3774 112.0398 106.0545 111.9277 106.3588 109.7657 111.5164 112.3433 105.7779 111.6098 105.7247 109.5023 183.0245 -63.1468 0.24840 0.01573 0.01510 −0.3421 1.4476 1.5524 2.1500 0.5706184 250 ± 30 583.5
1.519 1.519 1.0932 1.0932 1.9571 1.0905 1.0905 1.9571 1.0905 1.0905 109.8803 109.8902 109.8902 109.8902 109.8902 107.3633 110.5798 112.2432 105.9329 109.5329 105.9329 109.5329 110.5798 112.2432 105.9329 112.2432 105.9329 109.5329 180.0000 180.0000 0.41291 0.01259 0.01230 0 0 1.9722 1.9722 0.5684707 435 ± 70 1054.9
Bond distances are in Angstroms, bond angles are in degrees and dipole moments are in Debyes.
Fig. 3. Raman spectrum of 1,3-dibromopropane from 40 to 1500 cm−1 . Heavy trace, liquid; light trace, annealed crystal near 77 K.
Fig. 4. Raman spectrum of liquid 1,3-dibromopropane from 40 to 1500 cm−1 . Upper trace, parallel component; lower trace, perpendicular component.
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Table 3 Relative energies of the C2 , C1 and C2v conformers of 1,3-dibromobenzene as calculated using the basis sets listed Method/basis
C2 (GG)
C1 (AG)
C2v (AA)
rhf/6-31g* mp2 (full)/6-31g* mp2 (full)/6-31+g* mp2 (full)/6-31g(d,p) mp2 (full)/6-311g(d,p) mp2 (full)/6-311+g(d.p) mp2 (full)/6-311g(2d,2p) mp2 (full)/6-311+g(2d,2p) B3LYP/6-31g(d) B3LYP/6-31+g(d) B3LYP/6-311g(d) B3LYP/6-311+g(d) B3LYP/6-311g(d,p) B3LYP/6-311g(2d,2p) B3LYP/6-311+g(2d,2p) Experiment
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
349.8 583.5 840.9 834.4 309.3 349.4 367.9 389.4 433.9 558.1 202.0 186.7 225.9 224.9 214.5 250 ± 30
641.6 1054.9 745.7 728.0 485.8 549.3 609.1 619.8 781.0 353.3 333.0 307.4 378.1 520.2 358.7 435 ± 70
Remarks
Inverted Inverted
Inverted
Fig. 7. Calculated infrared spectra of 1,3-dibromopropane conformers from 400 to 1500 cm−1 : (A) C2v ; (B) C1 ; (C) C2 ; (D) composite spectrum for 56% C2 , 39% C1 and 5% C2v .
Energy units are cm−1 .
Fig. 8. Calculated Raman spectra of 1,3-dibromopropane conformers from 0 to 1500 cm−1 : (A) C2v ; (B) C1 ; (C) C2 ; (D) composite spectrum for 56% C2 , 39% C1 and 5% C2v .
Fig. 5. Raman spectrum of liquid 1,3-dibromopropane from 2800 to 3200 cm−1 . Upper trace, parallel component; lower trace, perpendicular component.
conformer were then applied without change to the calculation of the frequencies of the C1 and C2v conformers. The final scaling factors were 0.87 for the CH stretches, 0.885 for the CC and CBr stretches, and either 0.88 of 0.89 for most of the bending coordinates. The torsions were not well calculated by the Gaussian program and were left unscaled, as was the
Fig. 6. Internal coordinates used for normal coordinate calculations for 1,3dibromopropane. For clarity, the coordinates α1 , β2 , γ 1 , δ2 , µ2 and both bromomethyl torsions are omitted from the diagram.
CCC bend. This approach produced good agreement with the experimentally observed frequencies taken from condensed phase data. Tables 6–8 summarize the assignment of fundamentals for the three conformers together with the potential energy distribution for the final scaled normal coordinate calculation. Some of these results are displayed in Figs. 7–10. In Figs. 7 and 8, the approach used to synthesize the infrared and Raman spectrum of the liquid is illustrated. In Fig. 9, the infrared spectrum of the liquid at room temperature is compared with that calculated in Fig. 7. In Fig. 10, the Raman
Fig. 9. Comparison of experimental and calculated composite infrared spectrum: (A) composite; (B) experimental infrared spectrum of liquid 1,3dibromopropane.
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Fig. 10. Comparison of calculated Raman spectrum of the C2 conformer with experiment from 100 to 1500 cm−1 . Light line, calculated Raman spectrum of the C2 conformer; heavy line, Raman spectrum of the crystal near 77 K. The CH stretching region is shown in the inset.
spectrum of the crystal is compared with the Raman spectrum of the C2 conformer calculated using scaled rhf/6-31g* force constants and activities. The assignments were all made on the basis of agreement between the final scaled ab initio values of calculated frequency and the condensed phase infrared and especially Raman spectrum. The Raman spectrum of the annealed crystal was especially useful, agreeing almost quantitatively with the calculated Raman spectrum. When the assignments were complete for the major bands, it was apparent that the 424 cm−1 Raman band belonged to the C2 conformer and the 376 cm−1 band to the C1 conformer. Further, there were no other Raman bands of any conformer calculated to lie in this vicinity. The temperature dependence of the Raman spectrum of the liquid was then investigated, and it proved possible to extend the measurements to as low as −72 ◦ C, even though the melting point of the C2 conformer is known to be −34 ◦ C. The reason for this is, of course, that pure liquid 1,3-dibromopropane is a four component mixture, and the freezing point of the eutectic mixture may lie below −72 ◦ C; of course, it is also possible that insufficient time was allowed for crystals of the C2 conformer to nucleate. A plot of the ratio of the peak intensities of the 376 and 424 cm−1 bands is shown in Fig. 11, together with the least squares linear fit of the data. The slope of this line gives an enthalpy difference of 218.8 ± 14 cm−1 . This is probably the
Fig. 11. Plot of the logarithm of the peak intensity ratio for the 424 and 376 cm−1 Raman bands of liquid 1,3-dibromopropane vs. reciprocal absolute temperature. From the plot, H is 218.8 cm−1 for the C2 to C1 transition.
best measure of the C1 / C2 enthalpy difference afforded by these spectra. When the C2 CBr stretch at 549 cm−1 was ratioed to the C1 CBr stretch at 653 cm−1 , overlap with adjacent bands upset the measurement; the result was 279 ± 7 cm−1 . The larger intensities give rise to smaller statistical fluctuations, but the proximity of overlapping bands renders the measurement doubtful. Nonetheless, several such measurements were made and averaged, and the result was 250 ± 30 cm−1 . It became clear when the assignments were complete that there was only one band of the C2v conformer that was suitable for use in estimating the energy difference between the most stable C2 conformer and the C2v . This is the medium weak band with intensity maximum at 698–700 cm−1 in the Raman spectrum of the liquid. This band was compared to the C2 CCC stretch at 854 cm−1 , and the enthalpy difference was found to be 460 ± 14 cm−1 . Several other measures of these enthalpy differences were determined, resulting in a variety of enthalpy changes, averaging to about 250 ± 30 and 435 ± 60 cm−1 for the C2 –C1 and C2 –C2v transitions, respectively. The composition of the liquid at room temperature that is estimated from these conformational enthalpy differences is, in mole fractions, 0.60, 0.36 and 0.04 for the C2 , C1 and C2v conformer populations, respectively. These populations were assumed and a composite infrared and Raman spectrum constructed and compared with the experimental results. The comparison suggested that the C2 conformer was slightly overestimated at the expense of the other two, and the enthalpy difference for the C2 –C1 transition was changed from 251 to 220 cm−1 and that for the C2 –C2v to 376 cm−1 in order to recalculate the mole fractions. The results slightly modified the above to 0.56, 0.39 and 0.05 and the composite spectrum was recalculated with this composition. Agreement with experiment is excellent, as displayed in Figs. 9 and 10. Only the intensities in the CBr stretching region appear to be too small in the calculated spectrum, and the frequencies of the wagging modes in the 1300–1350 cm−1 region are not well calculated. The CH stretching region is unfortunately not well modeled by the calculations, as is shown in Fig. 10. However, it is clear from the low frequency results that only the C2 conformer survives in the pure crystal, and it is interesting that the overtone and combination tone of the CH2 deformations has a well-resolved doublet structure. It is not entirely forbidden that overtones and combination tones appear in crystal spectra, though it is rare. The reason is that such appearances require an essentially flat dispersion curve for the associated vibrations over an appreciable region of the first Brillouin zone. The k = 0 selection rule is then inapplicable, and the combination is allowed to appear in the observed spectrum. Otherwise, the region is understandable in terms of the spectrum of the C2 conformer, though the frequencies are somewhat displaced from agreement with experiment. The relative intensities are in at least crude agreement, and the calculation does predict the observed near coincidence of the antisymmetric CH stretching vibrations of the bromomethyl groups, and of their symmetric CH stretching frequencies
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Table 4 Observeda infrared and Raman bands of 1,3-dibromopropane Infrared
Raman
Gas
Rel. Int.
3028 B 3021 Q
vw w
2980 Q
m
Liquid
Rel. int.
3011
m
ms ms
2932.5 2920.5
ms mw, bd
2914 2932.6 2925 B
mw, bd w
Solid cryst.
Rel. int.
Liquid
Rel. int.
3023
w, sh
3019
m
3020
m
2973
vs, sh
2970.8
vs
2924.1
vs
2905.3
vs
2900
w, sh
2840.7 2832.9 2819.9 2812.7
w w m m
20.1
1430.9
vs
0.9
1425.2
m
1422.7 1417.9 1414.1 1355.3 1352.8 1344.2
m s vs m m m
1297.4 1289.2 1265 1243.9 1239.1 1234.2
w vs vw s vs vs
26.1 2967.1
2938.5 2935.5
Calc. int.b
s
2952 1.7
2905.4
ms
2870
mw
2848.5
m
2834
w
2818.6
mw
1444 R 1441 Q
s s
1453 1441 1431.5
w, sh m, sh vvs
1438 P
s
1419 Q
w, sh
1418.0
s
9.6
1357 B
w
w
m w mw vs
3.6
1300.5 B
1350.0 1340.4 1328.3 1293.6
23.9
1250 R 1247 Q 1243 P 1234.2
s s s vs
1239.6
vvs
68.9
1237 B 1196 B
w, sh vw
1194.8 1170.2 1123.4
mw w w
2.0
1086
vw
1078.6
w
1107.5 1073.7
vw ms
1074 1068
vw w
1051.6 1031.8
mw vw
1055 B
vvw 998.0
w
995 943.1
vw, sh ms
948 Q
m s
2970
0.8 0.3
1122.4 1117.1
w vw
2.9
1079.0
m
1074.7
m
0.0 6.4
959.9 945 943
m, w s vs
vs
Calc. int.b
Solid cryst.
νI
Assignment conformer and description
C2 CH2 Br antisymmetric CH stretch (B) C2 CH2 Br antisymmetric CH stretch (A) C2 CH2 Br symmetric CH stretch (B)
Rel. int.
42.2
3025
m
ν15
45.9
3023
sh
ν1
2973
vvs
ν16
33.1
2971
sh
ν2
C2 CH2 Br symmetric CH stretch (A)
97.8
2929, 2927
ν17
C2 antisymmetric CH2 stretch (B)
ν3
C2 symmetric stretch (A)
157.
m
2930 2909
wm, sh ms
2875 2851
vw, bd m
2821
vw
125
2908, 2904
mw
C2 ν5 (A) + ν18 (B)
2843 2835 2822 2815
C2 2 5 (A)
ν18
0.1 1435
ms
16.9
1419
ms
15.3
1341
w, bd
1293
m
CH2
1431
ms
1426 1420 1416 1356
m m ms vw
2.3
1346
vvw
2.7
1295 1291
vw, sh ms
3.8
1245
20.4
ν4
C2 CH2 Br deformation (B) C2 CH2 Br deformation (A)
ν5
C2 CH2 deformation (A)
ν19
C2 CH2 wag (B)
ν6 ν12
C2 CH2 Br wag (A) C1 CH2 Br wag
vw
ν20
C2 CH2 Br wag (B)
1237
ms
0.4 7.5
1125 1123
vw mw
ν7 ν13 ν14 ν21 ν8
C2 C1 C1 C2 C2
3.5
1080
w
15 ν22
C1 CH2 Br twist C2 antisymmetric CCC stretch (B)
1075
mw
1240
m, bd
1171 1125
vw w
1107 1075
vw w, br
1053 1030
mw w
ν16
C1 antisymmetric CCC stretch
998
mw
ν6
954 944
vw, br mw, br
C2v symmetric CCC stretch (A1 ) C2 CH2 Br rock (A) C2 CH2 rock (B)
1.8 0.3
961 945
w vw
ν9 ν23
CH2 Br twist (A) CH2 twist CH2 Br twist CH2 Br twist (B) CH2 twist (A)
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Table 4 (Continued ) Infrared
Raman
Gas
Rel. Int.
946 Q
m
905 854 B
vw, bd w
838 813 777 772 Q
w bd vw, bd s sh vs
770.5 Q 763 733 660 655 600 575 B 562 Q
vs vw, sh vw, bd vw, bd vvw, sh vw, bd vw vw
Liquid
Rel. int.
852.9
s
834.1
m
762.3
s
Calc. int.b
6.5
25.7
Solid cryst.
Rel. int.
856.3
s
852.9
s
760.3 758
vs vs
699.6 649.9 590.6
vvw s s
9.3
587.2 582
vs vw, sh
562.7 549.6
s s
10.9
424.3
vw
0.6
548.7 547 423 420
Liquid
Rel. int.
854
ms
834
vw
762
m, br
Calc. int.b
5.9
2.8
Solid cryst.
857
m
855
m
761 759
mw w, sh
700 651 590
mw s vvs
18.9
588
vvs
vs
564 551
vs s, sh
8.6
548
m
w
424
m, br
5.4
423 420
mw mw
6.7 1.3
376 313
m w, br
2.8 1.0
320 306
mw w
2.0 0.2 5.3
262 227 211
w, br m m
3.7 0.9 0.7
2.7
1.7 99
vvw
0.8
1.2
196 190 185 179 93 73 52
νI
Assignment conformer and description
ν18
C1 symmetric stretch
CCC
ν10
C2 symmetric stretch (A)
CCC
ν19
C1 CH2 Br rock
ν24
C2 CH2 Br rock (B)
ν20
C1 CH2 rock
ν7 ν21 ν11 ν26 ν22 ν25
C2v CBr stretch (A1 ) C1 CBr stretch C2 CBr stretch (A) C2v CBr stretch (B2 ) C1 CBr stretch C2 CBr stretch (B)
ν12
C2 CCC bend (A)
ν23 ν26
C1 CCBr bend C2 CCBr bend (B)
ν8 ν24 ν13
C2v CCC bend (A1 ) C1 CCBr bend C2 CCBr bend (A)
ν27
C2 CH2 Br torsion (B)
ν14
Lattice Lattice C2 CH2 Br torsion (A)?
Rel. int.
w mw w, sh w, b vw w vw
The symmetry species for each assignment is given in parentheses after the brief description in the last column. a Abbreviations: s, strong; m, medium; w, weak; v, very; b, broad; sh, shoulder; P, Q and R refer to the rotation–vibration branches of a gas phase infrared band; B refers to the center of a type B gas phase infrared band that lacks a central Q branch. b Intensity units used: infrared, km/mole; Raman, Angstrom4 /amu.
Table 5 Symmetry coordinates used in normal coordinate calculations C1 symmetry coordinate
C2
C2v
Descriptiona
S1 = r1 − r2 + r3 − r4 S2 = r1 − r2 − r3 + r4 S3 = r5 − r6 S4 = r1 + r2 + r3 + r4 S5 = r1 + r2 − r3 − r4 S6 = r5 + r6 √ √ √ √ S7 = (2 + 6)A + (2 − 6)B − α1 − α2 − β1 − β2 + (2 + 6)E + (2 − 6)M − ε1 − ε2 − µ1 − µ2 √ √ √ √ S8 = (2 + 6)A + (2 − 6)B − α1 − α2 − β1 − 2 − (2 + 6)E − (2 − 6)M + ε1 + ε2 + µ1 + µ2 √ √ S9 = (2 + 6)D + (2 − 6)G − γ 1 − γ 2 − δ1 − δ2 S10 = γ 1 + γ 2 − δ1 − δ2 S11 = α1 + α2 − β1 − β2 + ε1 + ε2 − µ1 − µ2 S12 = γ 1 − γ 2 − δ1 + δ2 S13 = α1 + α2 − β1 − β2 − ε1 − ε2 + µ1 + µ2 S14 = α1 − α2 − β1 + β2 + ε1 − ε2 − µ1 + µ2 S15 = α1 − α2 − β1 + β2 − ε1 + ε2 + µ1 − µ2 S16 = ρ1 − ρ2 S17 = ρ1 + ρ2
ν1 (A) ν15 (B) ν17 (B) ν2 (A) ν16 (B) ν3 (A) ν4 (A) ν18 (B) ν5 (A) ν19 (B) ν6 (A) ν7 (A) ν20 (B) ν8 (A) ν21 (B) ν22 (B) ν10 (A)
ν10 (A2 ) ν15 (B1 ) ν16 (B1 ) ν1 (A1 ) ν21 (B2 ) ν2 (A1 ) ν4 (A1 ) ν22 (B2 ) ν3 (A1 ) ν23 (B2 ) ν5 (A1 ) ν11 (A2 ) ν25 (B2 ) ν12 (A2 ) ν17 (B1 ) ν24 (B2 ) ν6 (A1 )
aI CH2 Br stretch ao CH2 Br stretch a CH2 stretch sI CH2 Br stretch so CH2 Br stretch s CH2 stretch I CH2 Br deformation o CH2 Br deformation CH2 deformation CH2 wag I CH2 Br wag CH2 twist o CH2 Br wag I CH2 Br twist o CH2 Br twist a CCC stretch s CCC stretch
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
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Table 5 (Continued ) C1 symmetry coordinate
C2
C2v
Descriptiona
S18 = α1 − α2 + β1 − β2 + ε1 − ε2 + µ1 − µ2 S19 = α1 − α2 + β1 − β2 − ε1 + ε2 − µ1 + µ2 S20 = γ 1 − γ 2 + δ1 − δ2 S21 = R1 − R2 S22 = R1 + R2 √ √ √ √ S23 = (2 − 6)A + (2 + 6)B − α1 − α2 − β1 − β2 − (2 − 6)E − (2 + 6)M + ε1 + ε2 + µ1 + µ2 √ √ S24 = (2 − 6)D − (2 + 6)G + γ 1 + γ 2 + δ1 + δ2 √ √ √ √ S25 = (2 − 6)A + (2 + 6)B − α1 − α2 − β1 − β2 + (2 − 6)E + (2 + 6)M − ε1 − ε2 − µ1 − µ2 S26 = τ 1 + τ 2 S27 = τ 1 − τ 2
ν9 (A) ν24 (B) ν23 (B) ν25 (B) ν11 (A) 26 (B) ν12 (A) ν13 (A) ν14 (A) ν27 (B)
ν13 (A2 ) ν18 (B1 ) ν19 (B1 ) ν26 (B2 ) ν7 (A1 ) ν27 (B2 ) ν8 (A1 ) ν9 (A1 ) ν14 (A2 ) ν20 (B1 )
I CH2 Br rock o CH2 Br rock CH2 rock a CBr stretch s CBr stretch o CCBr bend CCC bend I CCBr bend I torsion o torsion
Internal coordinates are displayed in Fig. 6. a Abbreviations: a, antisymmetric; s, symmetric; o, out of phase; I, in phase. Table 6 Assignment of fundamentals for the C2 (GG) conformer of 1,3-dibromopropane I
Description
Potential energy distribution
Wavenumber (cm−1 ) Obs.
Calc.
A ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14
CH2 Br stretch CH2 Br stretch CH2 stretch CH2 Br deformation CH2 deformation CH2 Br wag CH2 Br twist CH2 twist CH2 Br rock CCC stretch CBr stretch CCC bend CCBr bend CH2 Br torsion
99 S1 99 S4 100 S6 90 S7 , 7 S9 93 S9 , 7 S7 49 S11 , 46 S15 39 S14 , 28 S11 , 13 S12 37 S12 , 44 S14 , 17 S11 44 S18 , 25 S17 , 11 S14 , 11 S24 61 S17 , 34 S18 65 S22 , 17 S25 , 9 S24 54 S24 , 27 S22 , 12 S18 , 7 S25 73 S25 , 21 S24 100 S26
3020 2971 2905 1425 1419 1293 1240 1123 960 854 588 424 190 52
3029.7 2959.7 2892.0 1444.2 1432.1 1295.8 1245.6 1114.4 955.8 843.6 592.6 431.0 195.6 34.0
ν15 ν16 ν17 ν18 ν19 ν20 ν21 ν22 ν23 ν25 ν24 ν26 ν27
CH2 Br stretch CH2 Br stretch CH2 stretch CH2 Br deformation CH2 wag CH2 Br wag CH2 Br twist CCC stretch CH2 rock CH2 Br rock CBr stretch CCBr bend CH2 Br torsion
99 S2 91 S5 , 9 S3 91 S3 , 9 S5 98 S8 71 S10 , 12 S15 , 12 S16 88 S13 , 7 S20 66 S20 , 18 S16 57 S16 , 20 S10 , 13 S15 47 S20 , 27 S19 , 8 S16 67 S19 , 20 S20 , 5 S21 80 S21 , 10 S20 49 S23 , 30 S27 , 14 S21 , 6 S20 69 S27 , 31 S23
3025 2973 2929 1431 1341 1245 1125 1030 945 762 548 320 185
3030.0 2960.9 2943.0 1443.5 1350.0 1245,6 1116.0 1065.1 936.9 753.0 553.1 314.4 190.3
B
Abbreviations: Obs., observed; Calc., scaled calculated frequencies. Table 7 Assignment of fundamentals for the C1 (AG) conformer of 1,3-dibromopropane νI
Description
Potential energy distribution
Wavenumber (cm−1 ) Obs.
ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9
Antisymmetric CH2 Br stretch Antisymmetric CH2 Br stretch Antisymmetric CH2 stretch Symmetric CH2 Br stretch Symmetric CH2 Br stretch Symmetric CH2 stretch CH2 Br deformation CH2 deformation CH2 Br deformation
54 S1 , 41 S2 , 56 S2 , 42 S1 82 S3 , 11 S5 60 S4 , 32 S5 55 S5 , 38 S4 93 S6 46 S7 , 36 S9 , 15 S9 57 S8 , 35 S7 55 S9 , 27 S8 , 15 S7
Calc. 3029.4 3016.6 2966.2 2951.9 2946.7 2903.1 1452.0 1448.9 1442.8
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Table 7 (Continued ) νI
Description
ν10 ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18 ν19 ν20 ν21 ν22 ν23 ν25 ν24 ν26 ν27
Potential energy distribution
CH2 wag CH2 rock CH2 Br wag CH2 Br wag CH2 Br rock CH2 Br rock Antisymmetric CCC stretch Symmetric CCC stretch CH2 twist CH2 Br twist CH2 Br twist Symmetric CBr stretch Antisymmetric CBr stretch CCBr bend CCC bend CCBr bend CH2 Br torsion CH2 Br torsion
66 S10 , 9 S16 57 S11 , 20 S12 , 10 S15 33 S12 , 21 S14 , 19 S13 , 14 S18 62 S13 , 14 S12 , 10 S14 33 S14 , 22 S12 , 14 S10 , 9 S17 47 S15 , 18 S16 , 18 S11 63 S16 , 22 S15 28 S17 , 26 S20 , 25 S14 , 13 S18 23 S18 , 24 S17 , 12 S19 53 S19 , 23 S17 , 12 S20 33 S20 , 31 S18 , 21 S19 27 S21 , 27 S20 , 15 S24 , 10 S20 50 S22 , 32 S21 , 9 S25 27 S23 , 36 S21 , 20 S24 26 S24 , 39 S27 , 20 S23 62 S25 , 17 S23 , 9 S22 54 S26 , 20 S27 , 10 S24 , 9 S23 41 S27 , 43 S26
Wavenumber (cm−1 ) Obs.
Calc.
1341
1341.7 1287.3 1245.0 1230.7 1163.7 1095.8 1047.8 990.2 938.8 822.4 744.7 653.1 565.6 373.2 230.5 202.4 110.8 66.6
1171 1107 1030 998 (944) 835 (762) 651 564 376 227 211
Abbreviations: Obs., observed; Calc., scaled calculated frequencies.
Table 8 Assignment of fundamentals for the C2v (aa) conformer of 1,3-dibromopropane νI
Description
Potential energy distribution
Wavenumber (cm−1 ) Obs.
A1
A2
B1
B2
Calc.
ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9
CH2 Br stretch CH2 stretch CH2 deformation CH2 Br deformation CH2 Br wag CCC stretch CBr stretch CCC bend CCBr bend
95 S5 , 5 S6 95 S6 , 5 S5 87 S8 , 13 S7 85 S7 , 13 S8 97 S12 76 S17 , 12 S25 , 11 S24 58 S22 , 24 S24 , 11 S24 42 S24 , 43 S22 , 13 S17 75 S25 , 21 S24
ν10 ν11 ν12 ν13 ν14
CH2 Br CH stretch CH2 twist CH2 Br twist CH2 Br rock CH2 Br torsion
100 S1 71 S11 , 23 S14 77 S14 , 19 S11 89 S18 , 10 S11 100 S26
3010.7 1280.3 1073.5 782.8 120.3
ν15 ν16 ν17 ν18 ν19 ν20
CH2 Br CH stretch CH2 CH stretch CH2 Br twist CH2 Br rock CH2 rock CH2 Br torsion
89 S2 , 11 S3 89 S3 , 11 S2 66 S15 , 25 S20 42 S19 , 31 S15 , 27 S20 50 S20 , 48 S19 100 S27
3017.6 2977.2 1242.2 982.8 720.2 101.3
ν21 ν22 ν23 ν25 ν24 ν26 ν27
CH2 Br CH stretch CH2 Br deformation CH2 wag CH2 Br wag CCC stretch CBr stretch CCBr bend
100 S4 95 S9 65 S10 , 25 S13 70 S13 , 30 S10 92 S16 90 S21 , 10 S23 87 S23 , 12 S21
2942.9 1417.4 1327.8 1181.3 1027.2 588.2 314.7
Abbreviations: Obs., observed; Calc., scaled calculated frequencies.
698
1052
2947.1 2924.4 1464.2 1452.1 1247.1 1025.2 707.1 223.6 94.1
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
at a lower frequency. The symmetric and antisymmetric CH stretching vibrations of the methylene group occur at even lower frequencies, as assigned in Tables 5 and 6. Kanesaka et al. [10] recently showed that the crystal structure of 1,10-dibromodecane preserves the all anti structure in the crystal, and note that the same all anti structure has held true for several other longer chained alpha, omegadibromoalkanes, specifically 1,12-dibromododecane, [11] 1,16-dibromohexadecane [12] and 1,18-dibromooctadecane [13]. The crystal structure of 1,3-dibromopropane appears not to have been investigated, but the above data show that the structure stable in the crystal state is certainly not the all anti structure found for the longer molecules mentioned above. It should be interesting to see at what chain length the crystal structure accommodates the all anti structure, rather than the all gauche structure found in this work.
4. Conclusion The infrared and Raman spectrum of 1,3-dibromopropane has been reinvestigated in gas, liquid and crystalline solid forms. Ab initio calculations conducted in conjunction with scaled normal coordinate calculations have provided an excellent description of the infrared and Raman spectra of the liquid at room temperature. Assignments of fundamentals are nearly complete for the C2 (GG) conformer, and many assignments could be firmly made for the C1 (AG) conformer. Very few bands could be identified as originating in the C2v conformer, and the assignments are nearly incomplete for this. There was no evidence in the spectrum for the appearance of the Cs conformer.
Acknowledgement The authors are deeply grateful to the Dreyfus Foundation for grant SG-01-002, support that made possible the purchase
1557
of the air-cooled Ar+ ion laser, on which the Raman spectrum reported here is entirely dependent.
References [1] J. Thorbjornsrud, O.H. Ellestad, P. Klaboe, T. Torgrimson, J. Mol. Struct. 15 (1973) 61. [2] D.J. Duffy, J. Quenneville, T.M. Baumbaugh, S.A. Kitchener, R.K. McCormick, C.N. Dormady, T.A. Croce, A. Navabi, H.D. Stidham, S.L. Hsu, G.A. Guirgis, S. Deng, J.R. Durig, Spectrochem. Acta Part A 60 (2004) 659. [3] J.R. Durig, C. Zheng, M.A. Qtaitat, S. Deng, G.A. Guirgis, J. Mol. Struct. 657 (2003) 357. [4] J.R. Durig, C. Zheng, M.J. Williams, H.D. Stidham, G.A. Guirgis, Spectrochem. Acta Part A 60 (2004) 1659. [5] H.D. Stidham, D.J. Duffy, S.L. Hsu, G.A. Guirgis, J.R. Durig, Spectrochim. Acta Part A 57 (2001) 1567. [6] P.E. Farup, R. Stolevik, Acta Chem. Scand. A28 (1974) 680. [7] M.A. Cochran, A.S. Gilbert, J. Greer, R.A. Pethrick, Spectrochem. Acta Part A 32 (1976) 859. [8] G.A. Crowder, Spectrochem. Acta Part A 33 (1977) 721. [9] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malik, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian98, Revision A.3, Gaussian Inc., Pittsburgh, PA, 1998. [10] I. Kanesaka, S. Matsuzawa, T. Ishioka, Y. Kitagawa, Spectrochim. Acta Part A 60 (2004) 2621. [11] S. Kulpe, I. Seidel, K. Szulzewsky, U. Steger, E. Steger, Cryst. Res. Technol. 16 (1981) 349. [12] H. Kobayashi, T. Yamamoto, N. Nakamura, Cryst. Res. Technol. 30 (1995) 375. [13] N. Nakamura, Y. Yamamoto, H. Kobayashi, Y. Yoshimura, Cryst. Res. Technol. 28 (1993) 953.
Vibrational spectrum, ab initio calculation, conformational equilibria and torsional modes of 1,3-dibromopropane Matthew S. Nalewanski a, 1 , Yann P. Tambouret a, 1 , Scott T. Lentini a, 1 , Howard D. Stidham a, ∗ , Gamil A. Guirgis b b
a Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA Chemistry and Biochemistry Department, College of Charleston, Charleston, SC, USA
Received 26 October 2004; accepted 10 November 2004 Dedicated to Professor James R. Durig on the occasion of his 70th birthday as an excellent spectroscopist and great friend.
Abstract The infrared and Raman spectrum of 1,3-dibromopropane is reported in the crystalline, liquid and gaseous states. These measurements are compared to the results of ab initio calculations carried out using the 6-31+g* Gaussian basis set for a restricted Hartree–Fock computation. The calculation was repeated using second order Moeller–Ploesset perturbation theory to accommodate electron correlation using the 6-31g* basis set. The three most stable conformers are GG (C2 ), AG (C1 ) and AA (C2v ), where A and G stand for anti and gauche orientations of the bromomethyl group relative to the plane of the carbon atoms. The point group symmetry of each structure is given in parentheses. The fourth conformer, G G (Cs ) is of such high energy that it is not observed experimentally in isotropic media in either the infrared or Raman spectrum. In the crystalline state, comparison of the infrared and Raman spectrum with that calculated for the C2 conformer shows that only the GG (C2 ) conformer survives, and the doublet structure of many of the bands in the spectrum indicates at least two molecules per unit cell. The ab initio calculations predict and the temperature dependence of the Raman spectrum of the liquid confirms that the stability order is C2 < C1 < C2v Cs . These data show that in the liquid the C1 conformer lies 220 ± 30 cm−1 above the lowest energy C2 conformer, while the C2v is 435 ± 60 cm−1 above it. A nearly complete assignment of fundamentals is possible for the C2 conformer, and a number of bands can be confidently identified for the C1 . However, only two weak Raman bands with very weak infrared counterparts could be confidently assigned to the C2v conformer. A complete scaled normal coordinate calculation was conducted. The spectrum calculated from the results agrees very well with the observed. The average error for the C2 conformer could be reduced to as little as 0.6% (6.8 cm−1 ) by appropriate scaling. © 2004 Published by Elsevier B.V. Keywords: Vibrational spectrum; Ab initio calculations; Conformational equilibria 1,3-dibromopropane
1. Introduction The 1,3-dihalopropanes have been shown to have four spectroscopically distinct conformers [1–6]. If G and A refer to the gauche and anti orientations of the halomethyl groups with respect to the plane of the carbon atoms, these are GG (C2 ), AG (C1 ), AA (C2v ) and GG (Cs ), where the molecular point group of the isolated structure is indicated ∗ 1
Corresponding author. Tel.: +1 413 545 0048; fax: +1 413 545 4490. E-mail address: [email protected] (H.D. Stidham). Undergraduate participants.
1386-1425/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.saa.2004.11.060
in parentheses. These structures have statistical weights of 2, 4, 1 and 2, respectively. From electron diffraction measurements made on the gas at 35 ◦ C, the relative stabilities of 1,3-dibromopropane have been reported as 67% GG, 30% AG and 3% AA [6]. Since both bromine nuclei are on the same side of the plane of the carbon atoms in the GG conformer, this conformer has a much higher energy than the others and is not observed in the gas or liquid near room temperature [1,6]. A schematic drawing of the four conformers is shown in Fig. 1. In early work, Cochran et al. [7] reported spectroscopic work on 1,3-dibromopropane, but in later work Crowder
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M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
Fig. 1. Conformers of 1,3-dibromopropane.
[8] suggested that several fundamentals were misassigned. Thorbjornsrud et al. [1] then published an extensive study of the infrared and Raman spectra of several dihalo substituted propanes and assigned many of the fundamentals of the more stable conformers. The present work was undertaken as a logical extension of our earlier studies [2,4,5] in the expectation that a much more definitive assignment of the observed fundamentals could be made on the basis of a comparison of the infrared and Raman spectra in various states of aggregation with the results of ab initio and scaled normal coordinate calculation of the frequencies. Owing to limitations on the available computers, the calculations could not be performed at quite as high a level of the theory as was possible with the earlier studies. Normal coordinate calculations were done only with the C2 , C1 and C2v conformers using the results of rhf/6-31+g* and mp2 = full/6-31g* Gaussian98 [9] calculations. The scaled calculations, however, provide eerily accurate calculated spectra differing in the liquid state only slightly from the observed in only a few frequency or intensity details. This suggests that the composition of the liquid state at room temperature is nearly 56% C2 , 39% C1 and 5% C2v , in excellent agreement with the electron diffraction results of Farup and Stolevik [6], obtained for the gas at a slightly higher temperature.
2. Experimental 1,3-Dibromopropane (Sigma–Aldrich) was purified by trap to trap distillation on an all PyrexTM glass vacuum line, but was used otherwise as received. The purity stated by the supplier was 99%. Infrared spectra were obtained with a Mattson Cygnus 100 Fourier transform infrared interferometer operated at resolutions of 1 cm−1 for condensed phase and 0.5 cm−1 for the gas. Raman spectra were obtained with a Spex model II double monochromator equipped with 1200 grooves per mm gratings and a thermoelectrically cooled Hammamatsu R943-02 photomultiplier worked with locally designed photon counting electronics. Spectrometer advance and data collection
was done under computer control, using a National Instruments PCI-6602 dual counter–timer interface board operated with Labview VIs of local design to count photomultiplier pulses and drive the stepper motor that advances the spectrometer wavenumber setting. Raman spectra were excited by a Spectra-Physics Model 177-G42 air cooled Ar+ ion laser typically operated to deliver 100 mW of 514.5 nm continuous radiation at the laser head. Polarization measurements were made with a Polaroid analyzer set before a quartz polarization scrambler plate placed before the entrance slit of the monochromator. The Raman spectrum of the liquid was obtained by transillumination of PyrexTM glass capillaries filled and sealed under running vacuum while the sample was frozen in liquid nitrogen. Infrared and Raman spectra of the solids were obtained with evacuated all glass cold cells of local construction. The infrared cold cell was equipped with KBr windows and the sample was deposited under running vacuum onto a freshly polished CsI plate chilled to near liquid nitrogen temperatures. The Raman cold cell was equipped with a flat optical glass window. The sample was deposited on a brass block blackened to suppress laser reflections, by flaming an aqueous suspension of amorphous graphite onto the block, giving a surface very different from the CsI plate used in the infrared cold cell.
3. Results and discussion The results of the structural optimization for two of the ab initio calculations done are given in Tables 1 and 2. It is noteworthy that the restricted Hartree–Fock energy calculations using the 6-31+g* basis set reverses the order of stabilities for the C1 and C2v conformers. The Moeller–Ploesset full second order electron correlation (mp2) for the 6-31g* basis set retains the order, but predicts much too large energy differences. This was further investigated, and it was found that an inversion in the order was predicted by no other basis set. When the basis was specified as 6-31+g(d,p), the same inversion and essentially the same relative energy was found, and when a density functional calculation B3LYP for 6-31+g(d) basis was conducted, the inversion occurred again. The other basis sets tried are shown in Table 3, together with the relative energies of the conformers expressed in cm−1 units these produced. None of these inverted the relative energies, but also none calculated the experimentally observed well. To some extent, the differences between the highest level basis set results and experiment may be attributed to dipole–dipole interaction, which will be much more prevalent in the liquid than in the gas to which the ab initio results are appropriate. However, the calculated dipole moments of the various conformers indicate that only the Cs conformer ought to be appreciably stabilized by the interaction, since it has a calculated dipole moment of about 4 Debye. The three observable conformers have dipole moments of only 2–2.5 Debye (see for details Tables 1 and 2). There was nothing in these
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
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Table 1 Computational results for 1,3-dibromopropane using the rhf/6-31+g* basis Parameter
GG (C2 )
AG (C1 )
AA (C2v )
r(C1 –C2 ) r(C1 –C3 ) r(C1 –H4 ) r(C1 –H5 ) r(C2 –Br6 ) r(C2 –H7 ) r(C2 –H8 ) r(C3 –Br9 ) r(C3 –H10 ) r(C3 –H11 ) ∠(C2 –C1 –C3 ) ∠(C2 –C1 –H4 ) ∠(C3 –C1 –H4 ) ∠(C2 –C1 –H5 ) ∠(C3 –C1 –H5 ) ∠(H4 –C1 –H5 ) ∠(C1 –C2 –Br6 ) ∠(C1 –C2 –H7 ) ∠(Br6 –C2 –H7 ) ∠(C1 –C2 –H8 ) ∠(Br6 –C2 –H8 ) ∠(H7 –C2 –H8 ) ∠(C1 –C3 –Br9 ) ∠(C1 –C3 –H10 ) ∠(Br9 –C3 –H10 ) ∠(C1 –C3 –H11 ) ∠(Br9 –C3 –H11 ) ∠(H10 –C3 –H11 ) τ(C3 –C1 –C2 –Br6 ) τ(Br9 –C3 –C1 –C2 ) A (cm−1 ) B (cm−1 ) C (cm−1 ) |µa | (D) |µb | (D) |µc | (D) |µt | (D) (E+5256)(hartree) Raman relative energy, liquid (cm−1 ) rhf/6-31+g* relative energy, gas (cm−1 )
1.5182 1.5182 1.087 1.087 1.9594 1.078 1.077 1.9594 1.078 1.077 116.01 109.39 107.23 107.23 109.39 107.30 112.39 110.81 105.31 112.64 105.57 109.74 112.39 110.81 105.31 112.64 105.57 109.74 59.30 59.30 0.1479 0.0221 0.0207 0 0 2.9059 2.9059 0.9292469 0 0
1.5182 1.5205 1.0828 1.0869 1.9494 1.0792 1.0771 1.9605 1.0794 1.0781 112.30 110.47 110.15 109.22 107.18 107.34 111.28 111.98 105.58 112.17 105.83 109.62 112.70 112.75 105.81 110.97 104.87 109.32 −177.51 −69.68 0.2570 0.01526 0.01471 −0.2735 1.7325 1.7106 2.4501 0.9270859 250 ± 30 474
1.5205 1.5205 1.0831 1.0831 1.9468 1.0793 1.0793 1.9468 1.0793 1.0793 108.57 110.16 110.16 110.16 110.16 107.63 112.37 112.07 105.35 112.07 105.35 109.21 112.37 112.07 105.35 112.07 105.35 109.21 180.0 180.0 0.4592 0.01236 0.01213 0 0 2.1279 2.1279 0.9281609 435 ± 70 238
Bond distances are in Angstroms, bond angles are in degrees and dipole moments are in Debyes.
calculations that could identify the source of the inversions found, and the origin remains undetermined. The observed infrared and Raman spectrum in the gaseous, liquid and crystalline solid state are shown in Figs. 2–5. Experimentally observed frequencies are collected in Table 4, together with the assignments of fundamentals for the observed bands. The frequencies and infrared and Raman intensities were calculated using the basis sets of Tables 1 and 2. The force field in Cartesian coordinates was used to perform a normal coordinate calculation, using the internal coordinates quoted in Table 5, some of which are displayed in Fig. 6. When the frequencies calculated by the Gaussian suite of programs were reproduced, scaling was initiated. The scaling factors used for the final calculation were determined by matching experimental frequencies assigned to the C2 conformer as closely as possible. The scaling factors determined for the C2
Fig. 2. Infrared spectrum of 1,3-dibromopropane from 400 to 1500 cm−1 : (A) gas; (B) liquid; (C) annealed crystal near 77 K.
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Table 2 Computational results for 1,3-dibromopropane using the mp2 = full/6-31G* basis Parameter
GG (C2 )
r(C1 –C2 ) r(C1 –C3 ) r(C1 –H4 ) r(C1 –H5 ) r(C2 –Br6 ) r(C2 –H7 ) r(C2 –H8 ) r(C3 –Br9 ) r(C3 –H10 ) r(C3 –H11 ) ∠(C2 –C1 –C3 ) ∠(C2 –C1 –H4 ) ∠(C3 –C1 –H4 ) ∠(C2 –C1 –H5 ) ∠(C3 –C1 –H5 ) ∠(H4 –C1 –H5 ) ∠(C1 –C2 –Br6 ) ∠(C1 –C2 –H7 ) ∠(Br6 –C2 –H7 ) ∠(C1 –C2 –H8 ) ∠(Br6 –C2 –H8 ) ∠(H7 –C2 –H8 ) ∠(C1 –C3 –Br9 ) ∠(C1 –C3 –H10 ) ∠(Br9 –C3 –H10 ) ∠(C1 –C3 –H11 ) ∠(Br9 –C3 –H11 ) ∠(H10 –C3 –H11 ) τ(C3 –C1 –C2 –Br6 ) τ(Br9 –C3 –C1 –C2 ) A (cm−1 ) B (cm−1 ) C (cm−1 ) |µa |(D) |µb | (D) |µc | (D) |µt | (D) –(E + 5257)(hartree) Raman relative energy, liquid (cm−1 ) mp2/6-31g* relative energy, gas (cm−1 )
1.5155 1.5155 1.0966 1.0966 1.9611 1.0894 1.089 1.9611 1.0894 1.089 114.2052 109.4089 107.9747 107.9747 109.4089 107.6855 111.1308 111.7576 105.7923 111.783 105.9317 110.124 111.1308 111.7576 105.7923 111.783 105.9317 110.124 61.9951 61.9951 0.1555 0.0217 0.0205 0 0 2.6122 2.6122 0.5732771 0 0
AG (C1 )
AA (C2v )
1.516 1.5176 1.0935 1.0965 1.9566 1.0908 1.0883 1.9617 1.0907 1.0898 112.2494 109.7385 119.9874 109.3704 107.9299 107.4297 110.3774 112.0398 106.0545 111.9277 106.3588 109.7657 111.5164 112.3433 105.7779 111.6098 105.7247 109.5023 183.0245 -63.1468 0.24840 0.01573 0.01510 −0.3421 1.4476 1.5524 2.1500 0.5706184 250 ± 30 583.5
1.519 1.519 1.0932 1.0932 1.9571 1.0905 1.0905 1.9571 1.0905 1.0905 109.8803 109.8902 109.8902 109.8902 109.8902 107.3633 110.5798 112.2432 105.9329 109.5329 105.9329 109.5329 110.5798 112.2432 105.9329 112.2432 105.9329 109.5329 180.0000 180.0000 0.41291 0.01259 0.01230 0 0 1.9722 1.9722 0.5684707 435 ± 70 1054.9
Bond distances are in Angstroms, bond angles are in degrees and dipole moments are in Debyes.
Fig. 3. Raman spectrum of 1,3-dibromopropane from 40 to 1500 cm−1 . Heavy trace, liquid; light trace, annealed crystal near 77 K.
Fig. 4. Raman spectrum of liquid 1,3-dibromopropane from 40 to 1500 cm−1 . Upper trace, parallel component; lower trace, perpendicular component.
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
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Table 3 Relative energies of the C2 , C1 and C2v conformers of 1,3-dibromobenzene as calculated using the basis sets listed Method/basis
C2 (GG)
C1 (AG)
C2v (AA)
rhf/6-31g* mp2 (full)/6-31g* mp2 (full)/6-31+g* mp2 (full)/6-31g(d,p) mp2 (full)/6-311g(d,p) mp2 (full)/6-311+g(d.p) mp2 (full)/6-311g(2d,2p) mp2 (full)/6-311+g(2d,2p) B3LYP/6-31g(d) B3LYP/6-31+g(d) B3LYP/6-311g(d) B3LYP/6-311+g(d) B3LYP/6-311g(d,p) B3LYP/6-311g(2d,2p) B3LYP/6-311+g(2d,2p) Experiment
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
349.8 583.5 840.9 834.4 309.3 349.4 367.9 389.4 433.9 558.1 202.0 186.7 225.9 224.9 214.5 250 ± 30
641.6 1054.9 745.7 728.0 485.8 549.3 609.1 619.8 781.0 353.3 333.0 307.4 378.1 520.2 358.7 435 ± 70
Remarks
Inverted Inverted
Inverted
Fig. 7. Calculated infrared spectra of 1,3-dibromopropane conformers from 400 to 1500 cm−1 : (A) C2v ; (B) C1 ; (C) C2 ; (D) composite spectrum for 56% C2 , 39% C1 and 5% C2v .
Energy units are cm−1 .
Fig. 8. Calculated Raman spectra of 1,3-dibromopropane conformers from 0 to 1500 cm−1 : (A) C2v ; (B) C1 ; (C) C2 ; (D) composite spectrum for 56% C2 , 39% C1 and 5% C2v .
Fig. 5. Raman spectrum of liquid 1,3-dibromopropane from 2800 to 3200 cm−1 . Upper trace, parallel component; lower trace, perpendicular component.
conformer were then applied without change to the calculation of the frequencies of the C1 and C2v conformers. The final scaling factors were 0.87 for the CH stretches, 0.885 for the CC and CBr stretches, and either 0.88 of 0.89 for most of the bending coordinates. The torsions were not well calculated by the Gaussian program and were left unscaled, as was the
Fig. 6. Internal coordinates used for normal coordinate calculations for 1,3dibromopropane. For clarity, the coordinates α1 , β2 , γ 1 , δ2 , µ2 and both bromomethyl torsions are omitted from the diagram.
CCC bend. This approach produced good agreement with the experimentally observed frequencies taken from condensed phase data. Tables 6–8 summarize the assignment of fundamentals for the three conformers together with the potential energy distribution for the final scaled normal coordinate calculation. Some of these results are displayed in Figs. 7–10. In Figs. 7 and 8, the approach used to synthesize the infrared and Raman spectrum of the liquid is illustrated. In Fig. 9, the infrared spectrum of the liquid at room temperature is compared with that calculated in Fig. 7. In Fig. 10, the Raman
Fig. 9. Comparison of experimental and calculated composite infrared spectrum: (A) composite; (B) experimental infrared spectrum of liquid 1,3dibromopropane.
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Fig. 10. Comparison of calculated Raman spectrum of the C2 conformer with experiment from 100 to 1500 cm−1 . Light line, calculated Raman spectrum of the C2 conformer; heavy line, Raman spectrum of the crystal near 77 K. The CH stretching region is shown in the inset.
spectrum of the crystal is compared with the Raman spectrum of the C2 conformer calculated using scaled rhf/6-31g* force constants and activities. The assignments were all made on the basis of agreement between the final scaled ab initio values of calculated frequency and the condensed phase infrared and especially Raman spectrum. The Raman spectrum of the annealed crystal was especially useful, agreeing almost quantitatively with the calculated Raman spectrum. When the assignments were complete for the major bands, it was apparent that the 424 cm−1 Raman band belonged to the C2 conformer and the 376 cm−1 band to the C1 conformer. Further, there were no other Raman bands of any conformer calculated to lie in this vicinity. The temperature dependence of the Raman spectrum of the liquid was then investigated, and it proved possible to extend the measurements to as low as −72 ◦ C, even though the melting point of the C2 conformer is known to be −34 ◦ C. The reason for this is, of course, that pure liquid 1,3-dibromopropane is a four component mixture, and the freezing point of the eutectic mixture may lie below −72 ◦ C; of course, it is also possible that insufficient time was allowed for crystals of the C2 conformer to nucleate. A plot of the ratio of the peak intensities of the 376 and 424 cm−1 bands is shown in Fig. 11, together with the least squares linear fit of the data. The slope of this line gives an enthalpy difference of 218.8 ± 14 cm−1 . This is probably the
Fig. 11. Plot of the logarithm of the peak intensity ratio for the 424 and 376 cm−1 Raman bands of liquid 1,3-dibromopropane vs. reciprocal absolute temperature. From the plot, H is 218.8 cm−1 for the C2 to C1 transition.
best measure of the C1 / C2 enthalpy difference afforded by these spectra. When the C2 CBr stretch at 549 cm−1 was ratioed to the C1 CBr stretch at 653 cm−1 , overlap with adjacent bands upset the measurement; the result was 279 ± 7 cm−1 . The larger intensities give rise to smaller statistical fluctuations, but the proximity of overlapping bands renders the measurement doubtful. Nonetheless, several such measurements were made and averaged, and the result was 250 ± 30 cm−1 . It became clear when the assignments were complete that there was only one band of the C2v conformer that was suitable for use in estimating the energy difference between the most stable C2 conformer and the C2v . This is the medium weak band with intensity maximum at 698–700 cm−1 in the Raman spectrum of the liquid. This band was compared to the C2 CCC stretch at 854 cm−1 , and the enthalpy difference was found to be 460 ± 14 cm−1 . Several other measures of these enthalpy differences were determined, resulting in a variety of enthalpy changes, averaging to about 250 ± 30 and 435 ± 60 cm−1 for the C2 –C1 and C2 –C2v transitions, respectively. The composition of the liquid at room temperature that is estimated from these conformational enthalpy differences is, in mole fractions, 0.60, 0.36 and 0.04 for the C2 , C1 and C2v conformer populations, respectively. These populations were assumed and a composite infrared and Raman spectrum constructed and compared with the experimental results. The comparison suggested that the C2 conformer was slightly overestimated at the expense of the other two, and the enthalpy difference for the C2 –C1 transition was changed from 251 to 220 cm−1 and that for the C2 –C2v to 376 cm−1 in order to recalculate the mole fractions. The results slightly modified the above to 0.56, 0.39 and 0.05 and the composite spectrum was recalculated with this composition. Agreement with experiment is excellent, as displayed in Figs. 9 and 10. Only the intensities in the CBr stretching region appear to be too small in the calculated spectrum, and the frequencies of the wagging modes in the 1300–1350 cm−1 region are not well calculated. The CH stretching region is unfortunately not well modeled by the calculations, as is shown in Fig. 10. However, it is clear from the low frequency results that only the C2 conformer survives in the pure crystal, and it is interesting that the overtone and combination tone of the CH2 deformations has a well-resolved doublet structure. It is not entirely forbidden that overtones and combination tones appear in crystal spectra, though it is rare. The reason is that such appearances require an essentially flat dispersion curve for the associated vibrations over an appreciable region of the first Brillouin zone. The k = 0 selection rule is then inapplicable, and the combination is allowed to appear in the observed spectrum. Otherwise, the region is understandable in terms of the spectrum of the C2 conformer, though the frequencies are somewhat displaced from agreement with experiment. The relative intensities are in at least crude agreement, and the calculation does predict the observed near coincidence of the antisymmetric CH stretching vibrations of the bromomethyl groups, and of their symmetric CH stretching frequencies
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
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Table 4 Observeda infrared and Raman bands of 1,3-dibromopropane Infrared
Raman
Gas
Rel. Int.
3028 B 3021 Q
vw w
2980 Q
m
Liquid
Rel. int.
3011
m
ms ms
2932.5 2920.5
ms mw, bd
2914 2932.6 2925 B
mw, bd w
Solid cryst.
Rel. int.
Liquid
Rel. int.
3023
w, sh
3019
m
3020
m
2973
vs, sh
2970.8
vs
2924.1
vs
2905.3
vs
2900
w, sh
2840.7 2832.9 2819.9 2812.7
w w m m
20.1
1430.9
vs
0.9
1425.2
m
1422.7 1417.9 1414.1 1355.3 1352.8 1344.2
m s vs m m m
1297.4 1289.2 1265 1243.9 1239.1 1234.2
w vs vw s vs vs
26.1 2967.1
2938.5 2935.5
Calc. int.b
s
2952 1.7
2905.4
ms
2870
mw
2848.5
m
2834
w
2818.6
mw
1444 R 1441 Q
s s
1453 1441 1431.5
w, sh m, sh vvs
1438 P
s
1419 Q
w, sh
1418.0
s
9.6
1357 B
w
w
m w mw vs
3.6
1300.5 B
1350.0 1340.4 1328.3 1293.6
23.9
1250 R 1247 Q 1243 P 1234.2
s s s vs
1239.6
vvs
68.9
1237 B 1196 B
w, sh vw
1194.8 1170.2 1123.4
mw w w
2.0
1086
vw
1078.6
w
1107.5 1073.7
vw ms
1074 1068
vw w
1051.6 1031.8
mw vw
1055 B
vvw 998.0
w
995 943.1
vw, sh ms
948 Q
m s
2970
0.8 0.3
1122.4 1117.1
w vw
2.9
1079.0
m
1074.7
m
0.0 6.4
959.9 945 943
m, w s vs
vs
Calc. int.b
Solid cryst.
νI
Assignment conformer and description
C2 CH2 Br antisymmetric CH stretch (B) C2 CH2 Br antisymmetric CH stretch (A) C2 CH2 Br symmetric CH stretch (B)
Rel. int.
42.2
3025
m
ν15
45.9
3023
sh
ν1
2973
vvs
ν16
33.1
2971
sh
ν2
C2 CH2 Br symmetric CH stretch (A)
97.8
2929, 2927
ν17
C2 antisymmetric CH2 stretch (B)
ν3
C2 symmetric stretch (A)
157.
m
2930 2909
wm, sh ms
2875 2851
vw, bd m
2821
vw
125
2908, 2904
mw
C2 ν5 (A) + ν18 (B)
2843 2835 2822 2815
C2 2 5 (A)
ν18
0.1 1435
ms
16.9
1419
ms
15.3
1341
w, bd
1293
m
CH2
1431
ms
1426 1420 1416 1356
m m ms vw
2.3
1346
vvw
2.7
1295 1291
vw, sh ms
3.8
1245
20.4
ν4
C2 CH2 Br deformation (B) C2 CH2 Br deformation (A)
ν5
C2 CH2 deformation (A)
ν19
C2 CH2 wag (B)
ν6 ν12
C2 CH2 Br wag (A) C1 CH2 Br wag
vw
ν20
C2 CH2 Br wag (B)
1237
ms
0.4 7.5
1125 1123
vw mw
ν7 ν13 ν14 ν21 ν8
C2 C1 C1 C2 C2
3.5
1080
w
15 ν22
C1 CH2 Br twist C2 antisymmetric CCC stretch (B)
1075
mw
1240
m, bd
1171 1125
vw w
1107 1075
vw w, br
1053 1030
mw w
ν16
C1 antisymmetric CCC stretch
998
mw
ν6
954 944
vw, br mw, br
C2v symmetric CCC stretch (A1 ) C2 CH2 Br rock (A) C2 CH2 rock (B)
1.8 0.3
961 945
w vw
ν9 ν23
CH2 Br twist (A) CH2 twist CH2 Br twist CH2 Br twist (B) CH2 twist (A)
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M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
Table 4 (Continued ) Infrared
Raman
Gas
Rel. Int.
946 Q
m
905 854 B
vw, bd w
838 813 777 772 Q
w bd vw, bd s sh vs
770.5 Q 763 733 660 655 600 575 B 562 Q
vs vw, sh vw, bd vw, bd vvw, sh vw, bd vw vw
Liquid
Rel. int.
852.9
s
834.1
m
762.3
s
Calc. int.b
6.5
25.7
Solid cryst.
Rel. int.
856.3
s
852.9
s
760.3 758
vs vs
699.6 649.9 590.6
vvw s s
9.3
587.2 582
vs vw, sh
562.7 549.6
s s
10.9
424.3
vw
0.6
548.7 547 423 420
Liquid
Rel. int.
854
ms
834
vw
762
m, br
Calc. int.b
5.9
2.8
Solid cryst.
857
m
855
m
761 759
mw w, sh
700 651 590
mw s vvs
18.9
588
vvs
vs
564 551
vs s, sh
8.6
548
m
w
424
m, br
5.4
423 420
mw mw
6.7 1.3
376 313
m w, br
2.8 1.0
320 306
mw w
2.0 0.2 5.3
262 227 211
w, br m m
3.7 0.9 0.7
2.7
1.7 99
vvw
0.8
1.2
196 190 185 179 93 73 52
νI
Assignment conformer and description
ν18
C1 symmetric stretch
CCC
ν10
C2 symmetric stretch (A)
CCC
ν19
C1 CH2 Br rock
ν24
C2 CH2 Br rock (B)
ν20
C1 CH2 rock
ν7 ν21 ν11 ν26 ν22 ν25
C2v CBr stretch (A1 ) C1 CBr stretch C2 CBr stretch (A) C2v CBr stretch (B2 ) C1 CBr stretch C2 CBr stretch (B)
ν12
C2 CCC bend (A)
ν23 ν26
C1 CCBr bend C2 CCBr bend (B)
ν8 ν24 ν13
C2v CCC bend (A1 ) C1 CCBr bend C2 CCBr bend (A)
ν27
C2 CH2 Br torsion (B)
ν14
Lattice Lattice C2 CH2 Br torsion (A)?
Rel. int.
w mw w, sh w, b vw w vw
The symmetry species for each assignment is given in parentheses after the brief description in the last column. a Abbreviations: s, strong; m, medium; w, weak; v, very; b, broad; sh, shoulder; P, Q and R refer to the rotation–vibration branches of a gas phase infrared band; B refers to the center of a type B gas phase infrared band that lacks a central Q branch. b Intensity units used: infrared, km/mole; Raman, Angstrom4 /amu.
Table 5 Symmetry coordinates used in normal coordinate calculations C1 symmetry coordinate
C2
C2v
Descriptiona
S1 = r1 − r2 + r3 − r4 S2 = r1 − r2 − r3 + r4 S3 = r5 − r6 S4 = r1 + r2 + r3 + r4 S5 = r1 + r2 − r3 − r4 S6 = r5 + r6 √ √ √ √ S7 = (2 + 6)A + (2 − 6)B − α1 − α2 − β1 − β2 + (2 + 6)E + (2 − 6)M − ε1 − ε2 − µ1 − µ2 √ √ √ √ S8 = (2 + 6)A + (2 − 6)B − α1 − α2 − β1 − 2 − (2 + 6)E − (2 − 6)M + ε1 + ε2 + µ1 + µ2 √ √ S9 = (2 + 6)D + (2 − 6)G − γ 1 − γ 2 − δ1 − δ2 S10 = γ 1 + γ 2 − δ1 − δ2 S11 = α1 + α2 − β1 − β2 + ε1 + ε2 − µ1 − µ2 S12 = γ 1 − γ 2 − δ1 + δ2 S13 = α1 + α2 − β1 − β2 − ε1 − ε2 + µ1 + µ2 S14 = α1 − α2 − β1 + β2 + ε1 − ε2 − µ1 + µ2 S15 = α1 − α2 − β1 + β2 − ε1 + ε2 + µ1 − µ2 S16 = ρ1 − ρ2 S17 = ρ1 + ρ2
ν1 (A) ν15 (B) ν17 (B) ν2 (A) ν16 (B) ν3 (A) ν4 (A) ν18 (B) ν5 (A) ν19 (B) ν6 (A) ν7 (A) ν20 (B) ν8 (A) ν21 (B) ν22 (B) ν10 (A)
ν10 (A2 ) ν15 (B1 ) ν16 (B1 ) ν1 (A1 ) ν21 (B2 ) ν2 (A1 ) ν4 (A1 ) ν22 (B2 ) ν3 (A1 ) ν23 (B2 ) ν5 (A1 ) ν11 (A2 ) ν25 (B2 ) ν12 (A2 ) ν17 (B1 ) ν24 (B2 ) ν6 (A1 )
aI CH2 Br stretch ao CH2 Br stretch a CH2 stretch sI CH2 Br stretch so CH2 Br stretch s CH2 stretch I CH2 Br deformation o CH2 Br deformation CH2 deformation CH2 wag I CH2 Br wag CH2 twist o CH2 Br wag I CH2 Br twist o CH2 Br twist a CCC stretch s CCC stretch
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
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Table 5 (Continued ) C1 symmetry coordinate
C2
C2v
Descriptiona
S18 = α1 − α2 + β1 − β2 + ε1 − ε2 + µ1 − µ2 S19 = α1 − α2 + β1 − β2 − ε1 + ε2 − µ1 + µ2 S20 = γ 1 − γ 2 + δ1 − δ2 S21 = R1 − R2 S22 = R1 + R2 √ √ √ √ S23 = (2 − 6)A + (2 + 6)B − α1 − α2 − β1 − β2 − (2 − 6)E − (2 + 6)M + ε1 + ε2 + µ1 + µ2 √ √ S24 = (2 − 6)D − (2 + 6)G + γ 1 + γ 2 + δ1 + δ2 √ √ √ √ S25 = (2 − 6)A + (2 + 6)B − α1 − α2 − β1 − β2 + (2 − 6)E + (2 + 6)M − ε1 − ε2 − µ1 − µ2 S26 = τ 1 + τ 2 S27 = τ 1 − τ 2
ν9 (A) ν24 (B) ν23 (B) ν25 (B) ν11 (A) 26 (B) ν12 (A) ν13 (A) ν14 (A) ν27 (B)
ν13 (A2 ) ν18 (B1 ) ν19 (B1 ) ν26 (B2 ) ν7 (A1 ) ν27 (B2 ) ν8 (A1 ) ν9 (A1 ) ν14 (A2 ) ν20 (B1 )
I CH2 Br rock o CH2 Br rock CH2 rock a CBr stretch s CBr stretch o CCBr bend CCC bend I CCBr bend I torsion o torsion
Internal coordinates are displayed in Fig. 6. a Abbreviations: a, antisymmetric; s, symmetric; o, out of phase; I, in phase. Table 6 Assignment of fundamentals for the C2 (GG) conformer of 1,3-dibromopropane I
Description
Potential energy distribution
Wavenumber (cm−1 ) Obs.
Calc.
A ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14
CH2 Br stretch CH2 Br stretch CH2 stretch CH2 Br deformation CH2 deformation CH2 Br wag CH2 Br twist CH2 twist CH2 Br rock CCC stretch CBr stretch CCC bend CCBr bend CH2 Br torsion
99 S1 99 S4 100 S6 90 S7 , 7 S9 93 S9 , 7 S7 49 S11 , 46 S15 39 S14 , 28 S11 , 13 S12 37 S12 , 44 S14 , 17 S11 44 S18 , 25 S17 , 11 S14 , 11 S24 61 S17 , 34 S18 65 S22 , 17 S25 , 9 S24 54 S24 , 27 S22 , 12 S18 , 7 S25 73 S25 , 21 S24 100 S26
3020 2971 2905 1425 1419 1293 1240 1123 960 854 588 424 190 52
3029.7 2959.7 2892.0 1444.2 1432.1 1295.8 1245.6 1114.4 955.8 843.6 592.6 431.0 195.6 34.0
ν15 ν16 ν17 ν18 ν19 ν20 ν21 ν22 ν23 ν25 ν24 ν26 ν27
CH2 Br stretch CH2 Br stretch CH2 stretch CH2 Br deformation CH2 wag CH2 Br wag CH2 Br twist CCC stretch CH2 rock CH2 Br rock CBr stretch CCBr bend CH2 Br torsion
99 S2 91 S5 , 9 S3 91 S3 , 9 S5 98 S8 71 S10 , 12 S15 , 12 S16 88 S13 , 7 S20 66 S20 , 18 S16 57 S16 , 20 S10 , 13 S15 47 S20 , 27 S19 , 8 S16 67 S19 , 20 S20 , 5 S21 80 S21 , 10 S20 49 S23 , 30 S27 , 14 S21 , 6 S20 69 S27 , 31 S23
3025 2973 2929 1431 1341 1245 1125 1030 945 762 548 320 185
3030.0 2960.9 2943.0 1443.5 1350.0 1245,6 1116.0 1065.1 936.9 753.0 553.1 314.4 190.3
B
Abbreviations: Obs., observed; Calc., scaled calculated frequencies. Table 7 Assignment of fundamentals for the C1 (AG) conformer of 1,3-dibromopropane νI
Description
Potential energy distribution
Wavenumber (cm−1 ) Obs.
ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9
Antisymmetric CH2 Br stretch Antisymmetric CH2 Br stretch Antisymmetric CH2 stretch Symmetric CH2 Br stretch Symmetric CH2 Br stretch Symmetric CH2 stretch CH2 Br deformation CH2 deformation CH2 Br deformation
54 S1 , 41 S2 , 56 S2 , 42 S1 82 S3 , 11 S5 60 S4 , 32 S5 55 S5 , 38 S4 93 S6 46 S7 , 36 S9 , 15 S9 57 S8 , 35 S7 55 S9 , 27 S8 , 15 S7
Calc. 3029.4 3016.6 2966.2 2951.9 2946.7 2903.1 1452.0 1448.9 1442.8
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M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
Table 7 (Continued ) νI
Description
ν10 ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18 ν19 ν20 ν21 ν22 ν23 ν25 ν24 ν26 ν27
Potential energy distribution
CH2 wag CH2 rock CH2 Br wag CH2 Br wag CH2 Br rock CH2 Br rock Antisymmetric CCC stretch Symmetric CCC stretch CH2 twist CH2 Br twist CH2 Br twist Symmetric CBr stretch Antisymmetric CBr stretch CCBr bend CCC bend CCBr bend CH2 Br torsion CH2 Br torsion
66 S10 , 9 S16 57 S11 , 20 S12 , 10 S15 33 S12 , 21 S14 , 19 S13 , 14 S18 62 S13 , 14 S12 , 10 S14 33 S14 , 22 S12 , 14 S10 , 9 S17 47 S15 , 18 S16 , 18 S11 63 S16 , 22 S15 28 S17 , 26 S20 , 25 S14 , 13 S18 23 S18 , 24 S17 , 12 S19 53 S19 , 23 S17 , 12 S20 33 S20 , 31 S18 , 21 S19 27 S21 , 27 S20 , 15 S24 , 10 S20 50 S22 , 32 S21 , 9 S25 27 S23 , 36 S21 , 20 S24 26 S24 , 39 S27 , 20 S23 62 S25 , 17 S23 , 9 S22 54 S26 , 20 S27 , 10 S24 , 9 S23 41 S27 , 43 S26
Wavenumber (cm−1 ) Obs.
Calc.
1341
1341.7 1287.3 1245.0 1230.7 1163.7 1095.8 1047.8 990.2 938.8 822.4 744.7 653.1 565.6 373.2 230.5 202.4 110.8 66.6
1171 1107 1030 998 (944) 835 (762) 651 564 376 227 211
Abbreviations: Obs., observed; Calc., scaled calculated frequencies.
Table 8 Assignment of fundamentals for the C2v (aa) conformer of 1,3-dibromopropane νI
Description
Potential energy distribution
Wavenumber (cm−1 ) Obs.
A1
A2
B1
B2
Calc.
ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9
CH2 Br stretch CH2 stretch CH2 deformation CH2 Br deformation CH2 Br wag CCC stretch CBr stretch CCC bend CCBr bend
95 S5 , 5 S6 95 S6 , 5 S5 87 S8 , 13 S7 85 S7 , 13 S8 97 S12 76 S17 , 12 S25 , 11 S24 58 S22 , 24 S24 , 11 S24 42 S24 , 43 S22 , 13 S17 75 S25 , 21 S24
ν10 ν11 ν12 ν13 ν14
CH2 Br CH stretch CH2 twist CH2 Br twist CH2 Br rock CH2 Br torsion
100 S1 71 S11 , 23 S14 77 S14 , 19 S11 89 S18 , 10 S11 100 S26
3010.7 1280.3 1073.5 782.8 120.3
ν15 ν16 ν17 ν18 ν19 ν20
CH2 Br CH stretch CH2 CH stretch CH2 Br twist CH2 Br rock CH2 rock CH2 Br torsion
89 S2 , 11 S3 89 S3 , 11 S2 66 S15 , 25 S20 42 S19 , 31 S15 , 27 S20 50 S20 , 48 S19 100 S27
3017.6 2977.2 1242.2 982.8 720.2 101.3
ν21 ν22 ν23 ν25 ν24 ν26 ν27
CH2 Br CH stretch CH2 Br deformation CH2 wag CH2 Br wag CCC stretch CBr stretch CCBr bend
100 S4 95 S9 65 S10 , 25 S13 70 S13 , 30 S10 92 S16 90 S21 , 10 S23 87 S23 , 12 S21
2942.9 1417.4 1327.8 1181.3 1027.2 588.2 314.7
Abbreviations: Obs., observed; Calc., scaled calculated frequencies.
698
1052
2947.1 2924.4 1464.2 1452.1 1247.1 1025.2 707.1 223.6 94.1
M.S. Nalewanski et al. / Spectrochimica Acta Part A 61 (2005) 1547–1557
at a lower frequency. The symmetric and antisymmetric CH stretching vibrations of the methylene group occur at even lower frequencies, as assigned in Tables 5 and 6. Kanesaka et al. [10] recently showed that the crystal structure of 1,10-dibromodecane preserves the all anti structure in the crystal, and note that the same all anti structure has held true for several other longer chained alpha, omegadibromoalkanes, specifically 1,12-dibromododecane, [11] 1,16-dibromohexadecane [12] and 1,18-dibromooctadecane [13]. The crystal structure of 1,3-dibromopropane appears not to have been investigated, but the above data show that the structure stable in the crystal state is certainly not the all anti structure found for the longer molecules mentioned above. It should be interesting to see at what chain length the crystal structure accommodates the all anti structure, rather than the all gauche structure found in this work.
4. Conclusion The infrared and Raman spectrum of 1,3-dibromopropane has been reinvestigated in gas, liquid and crystalline solid forms. Ab initio calculations conducted in conjunction with scaled normal coordinate calculations have provided an excellent description of the infrared and Raman spectra of the liquid at room temperature. Assignments of fundamentals are nearly complete for the C2 (GG) conformer, and many assignments could be firmly made for the C1 (AG) conformer. Very few bands could be identified as originating in the C2v conformer, and the assignments are nearly incomplete for this. There was no evidence in the spectrum for the appearance of the Cs conformer.
Acknowledgement The authors are deeply grateful to the Dreyfus Foundation for grant SG-01-002, support that made possible the purchase
1557
of the air-cooled Ar+ ion laser, on which the Raman spectrum reported here is entirely dependent.
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