W.G. Fateley

Torsional frequencies in the far infrared—IV. Torsions around the CC single bond in conjugated molecules

Abstract Torsions around the central single bond in conjugated molecules have been studied in the far infrared. Twelve derivatives of butadiene and glyoxal, plus benzaldehyde and styrene, were examined in the gas phase, and the frequency was located for eleven of the compounds. For benzaldehyde and acrolein there are very large frequency increases upon going from gas to liquid, with δv/v equal to 15–20 per cent. Using the potential function 2V = 3Σn=1 Vn (1 - cos nα), the quantity [V1 + 4V2 + 9V3] was evaluated for all eleven compounds. For benzaldehyde, V2 (gas) = 1630 cm−1 or 4·6 kcal/mole; V2, (liquid) = 2340 cm−1 or 6·7 kcal/mole. A method is described for evaluating V1, V2 and V3 separately if appropriate data are available. It is applied to acrolein, and tentative values for these constants are deduced.

Torsional frequencies in the far infrared—V. Torsions around the CC signle bond in some benzaldehydes, furfural, and related compounds☆

Abstract The infrared spectrum between 33 and 400 cm−1 has been examined in the vapor and liquid phases for the following 26 compounds: o-, m-, and p-F, Cl, Br, and CH3 benzaldehydes; pyridine-2-, -3-, and 4-aldehydes; acetophenone and its o-, m-, and p- F derivatives; furan-2-aldehyde; and several monofluorostyrenes and -nitrobenzenes. Torsional frequencies were sought, and have been assigned for all but the last two groups of compounds. For all the meta-substituted benzaldehydes, for a few of the ortho ones, and for furan-2-aldehyde evidence has been found for the presence of two rotational isomers in the vapor. The parameters V1 and V2 of an approximate potential function for the internal rotation have been evaluated. In the meta benzaldehydes the O-cis rotamer is the more stable one, whereas in the ortho compounds and in furan-2-aldehyde it is the O-trans one. For meta-fluorobenzaldehyde a temperature-dependence study of some bands in the mid-infrared has confirmed the existence of two rotamers, and has given 0.5 kcal/mole as an approximate value for their energy difference in CDCl3 solution.

Torsional frequencies in the far infrared—III: The form of the potential curve for hindered internal rotation of a methyl group

Abstract Transitions between excited torsional levels (0 → 1, 1 → 2, 2 → 3, ...) have been measured in the infrared for several compounds. The observed frequencies provide the best test of the potential function for hindered internal rotation which has yet been made. The constants of the usual expression Thus V 6 ⩽ 3 per cent of V 3 and may be either positive or negative. Pitzer and Hollenbergs earlier results for CH 3 CCl 3 could not be confirmed. The spectra for all six compounds were examined from 100 to 435 cm −1 , allowing some bending frequencies to be observed and tabulated.

Far infrared spectra of dimeric and crystalline formic and acetic acids

Infrared spectra of gaseous (HCOOH)2, (HCOOD)2, (CH3COOH)2, and (CD3COOH)2 have been obtained over the range of 33–400 cm−1. All of the expected low frequency, infrared-active “hydrogen bond” fundamentals were located and assigned. The spectra of crystalline, polymeric HCOOH and HCOOD show some structure in the 250 cm−1 region but do not verify the interpretation of the earlier Raman data. For crystalline CH3COOH only a single intense band at 198 cm−1 was observed.

The infrared spectrum of carbon suboxide

Abstract The infrared spectrum of gaseous and solid C 3 O 2 has been examined from 70 to 4000 cm −1 . Because impurity bands have been a problem in the past, strenuous efforts were made to obtain an authentic spectrum. Samples were prepared by three different methods. (1) by dehydration of malonic acid with P 2 O 5 , (2) by dehydration of malonic acid-d 4 with P 2 O 5 , and (3) by pyrolysis of diacetyl tartaric anhydride. All three gave samples whose infrared spectra were quantitatively identical. In addition only traces of impurities were found by gas chromatography. The spectrum reported here is therefore surely that of pure C 3 O 2 . The range 70–400 cm −1 was examined with very thick gas and solid samples in an effort to locate the lowest fundamental. It was not observed, and is surely outside this region. The spectrum of the gas from 400 to 4000 cm −1 was examined with a resolution of 0·5–1 cm −1 . Six of the seven fundamentals for the linear form are known with certainty, and the seventh is believed to be below 70 cm −1 . Nonetheless there are many features of the spectrum which are still unexplained, and the analysis is far from satisfactory.

The measurement of conformational equilibria via the infrared studies of 1,1-dibromo-3-fluorobutadiene-1,3 and 1,1-dichloro-3-fluorobutadiene-1,3

Abstract In the analysis of conformational equilibria it is customarily assumed that the absorption coefficients of the infrared bands are temperature independent. In this study a convenient test of this assumption is proposed which also provides for the immediate calculation of the equilibrium constant. The method is applied to the s-trans ⇌ skew equilibria of 1,1-dibromo-3-fluorobutadiene-1,3 and 1,1-dichloro-3-fluorobutadiene-1,3. For the former compound ΔH = 844 ± 100 cal/mole and ΔS = 3.5 ± 1 e.u. and for the latter ΔH = 706 ± 100 cal/mole and ΔS = 2.3 ± 1 e.u. in CS2 solvent. Some approximations used in previous studies axe briefly discussed and the present method is applied to the data presented in one of these reports.

Vapor-liquid frequency shifts in the low-frequency infrared spectrum

Abstract Data are given to show that in the low frequency region of the infrared spectrum (below about 300 cm−1), gas-phase frequencies are usually less than liquid-phase ones. This is the exact opposite of the behavior in the region above 700 cm−1. Some implications of this are discussed.

Torsional frequencies in the far infrared—I: Molecules with a single methyl rotor*†

Abstract Transitions between torsional levels have been observed in the far infrared for nine compounds. The results are: Compound cm -1 v ″ → v ′ Potential barrier (cal/mole) CH 3 CHO 150 0→1 ( A ) 1180 262 0→2 ( A ) 1186 276 0→2 ( E ) 1179 CH 3 CH 2 Cl 243 0→1 ( A ) 3580 CH 3 CHF 2 222 0→1 ( A ) 3210 HCOOCH 3 130 0→1 ( A ) 1165 CH 3 N 3 126 ? ? CD 3 N 3 90 ? ? CH 3 NCO 143 ? ? CH 3 NCS 128 ? ? CH 3 SCN 131 ? ? The potential barriers for the first four compounds are in excellent agreement with microwave results. For the remaining five substances there are questions about which torsional transition is being observed, and there are no good microwave results for comparison.

Far-Infrared Transmission of Commercially Available Crystals and High-Density Polyethylene

Recently, far-infrared spectrometers and interferometers have become available to many researchers and laboratories. With this new interest in the low-energy region of the spectrum, we thought it useful to publish a collection of transmission curves of commercially available crystals and high-density polyethylene so that choices could be made for cell windows and filters for work in this region.1