Group Frequency Correlations Research Articles

Interpretation of the vibrational spectra of bicyclic hydrocarbons: norbornane and norbornadiene

Abstract Infrared spectra of gaseous and polycrystalline norbornane [Ricyclo (2.2.1) heptane] and norbornadiene [bicyclo (2.2.1) hepta-2,5-diene] were recorded from 300 to 4000 cm −1 . Raman spectra for the liquid and solid states of both systems were measured from 100 to 4000 cm −1 . Vibrational assignments for these two appreciably strained bicyclic molecules were based upon group frequency correlations, Raman and infrared band intensities and Raman depolarization ratios. Low temperature Raman techniques appear particularly useful in the characterization and assignment of the fundamental vibrations of these systems. Specifically, the observation of weak transitions and the analyses of diffuse spectral regions are facilitated by investigations at liquid nitrogen temperatures. Normal coordinate analyses were performed at various stages of the assignment process in order to access the consistency of the spectral assignments and to identify strongly coupled modes. In these calculations particular emphasis was directed toward the vibrational descriptions provided by the potential energy distributions.

Interpretation of the vibrational spectra of small ring systems: I. 1,1,2,2-Tetrachlorotetrafluorocyclobutane and 1-chloro-2,2,3,3-tetrafluorocyclobutane

The infrared spectra of the vapor and crystalline states of cyclo-C 4Cl 4F 4 and cyclo-C 4ClF 4H 3 have been measured between 200 and 4000 cm −1. The low temperature infrared spectrum of each molecule has also been studied down to 33 cm −1. A weak band at 102 cm −1 in the infrared spectrum of polycrystalline c-C 4ClF 4H 3 has been assigned to the ring-puckering fundamental; however, the corresponding vibration for c-C 4Cl 4F 4 was not observed. The Raman spectra of each compound have been recorded at ambient and liquid nitrogen temperatures to within 50 cm −1 of the exciting line. Although it has not been possible to establish the equilibrium structure of the c-C 4ClF 4H 3 skeleton from the infrared and Raman data, the spectra of c-C 4Cl 4F 4 are explained in terms of C 2v molecular symmetry. A vibrational assignment is suggested for each molecule on the basis of relative band intensities, group frequency correlations and depolarization ratios.

Interpretation of the infrared and laser-Raman spectra of cyclopentene and perfluorocyclopentene

The infrared spectra of gaseous and polycrystalline cyclopentene and perfluorocyclopentene have been recorded from 3500 to 200 cm −1. Raman spectra of the gaseous, liquid and crystalline states for the respective molecules have also been studied and depolarization values determined. In addition, the farinfrared spectrum of solid perfluorocyclopentene has been recorded from 33 to 300 cm −1. The 33 normal modes of each molecule have been assigned on the basis of the measured depolarization ratios, relative band intensities and group frequency correlations. A series of lines associated with the Δ v = 2 transitions of the ring-puckering vibration of gaseous cyclopentene were readily detected in the Raman spectrum. The ring-puckering fundamental of perfluorocyclopentene is assigned to the Raman line centered at 100 cm −1. The data for these molecules will be discussed in conjunction with similar studies on other cyclic systems.

Spectra and structure of small ring compounds. XXXVI. 2‐methyl‐1,3‐dioxolane; 2‐methyl‐1,3‐dioxolane‐d4; 2‐methyltetrahydrofuran; and methylcyclopentane

AbstractThe infrared spectra of solid, liquid and gaseous 2‐methyl‐1,3‐dioxolane, 2‐methyl‐1,3‐dioxolane‐d4, 2‐methyltetrahydrofuran, and methylcyclopentane were recorded from 4000 to 33 cm−1. The Raman spectra were also recorded and depolarization values were measured for 2‐methyl‐1,3‐dioxolane and methylcyclopentane. The fundamental vibrations of each compound have been assigned on the basis of group frequency correlations, isotopic shifts, and band intensities as well as depolarization values when applicable. The barrier hindering internal rotation of the methyl group has been calculated to be 3.5, 3.6, and 4.2 kcal mole−1 for 2‐methyl‐1,3‐dioxolane, 2‐methyltetrahydrofuran, and methylcyclopentane, respectively. The pseudorotational ring vibration was observed at 58 and 56 cm−1 for gaseous 2‐methyl‐1,3‐dioxolane and 2‐methyltetrahydrofuran, respectively. This implies a barrier to pseudorotation of the order of 1.3 kcal mole−1 for 2‐methyl‐1,3‐dioxolane and 2‐methyl‐tetrahydrofuran. The pseudorotational mode could not be observed for methylcyclopentane, even at maximum pressure‐path length condition.

Vibrational Spectra and Structure of 3-Methyl-1,2-Butadiene

The infrared and Raman spectra of gaseous and crystalline 3-methyl-1,2-butadiene have been recorded from 140 to 4000 cm−1. All spectra have been interpreted in detail, and the fundamentals have been assigned on the basis of depolarization ratios, vapor-phase band contours, relative band intensities, and group frequency correlations. The interesting a2 and b2 methyl torsional vibrations are assigned to bands at 185 and 236 cm−1, respectively, in the spectrum recorded at liquid nitrogen temperatures. From these frequencies the magnitude of the internal rotational barrier is estimated to be 2.7 kcal/mole. The barrier height and the observed top-top frequency splitting is discussed in conjunction with recent studies on similar molecules.

Vibrational Spectra and Structure of Acetone Oxime and Acetone Oxime-O-d

The infrared and Raman spectra of acetone oxime have been investigated from 70 to 4000 cm−1. Spectra have been obtained for the gaseous, liquid, and polycrystalline materials, and the data have been interpreted in detail. A suggested vibrational assignment for the molecule is based upon group frequency correlations, relative band intensities, and isotopic and vapor-solid frequency shifts. The interesting methyl torsional vibrations were assigned to a band at 217 cm−1 in the infrared spectrum recorded at liquid nitrogen temperatures. The magnitude of the internal rotational barrier for solid acetone oxime is estimated to be 3.0 kcal/mole. The results of this analysis are considered in conjunction with previous studies on similar molecules.

Vibrational Spectra and Structure of Azomethane and Azomethane-d6

The infrared spectra of gaseous and polycrystalline CH3NNCH3 and CD3NNCD3 have been recorded from 33 to 4000 cm−1. The corresponding Raman spectra of the liquid and solid samples have been measured from 100 to 4000 cm−1. The vibrational data provide definitive verification of a trans structure with C2h symmetry. A vibrational assignment is proposed for the 24 fundamental modes based upon the observed band contours, isotopic shift ratios, and group frequency correlations. The au and bg torsional modes were assigned to bands at 222 and 223 cm−1, respectively, and the internal rotational barrier of the methyl group was calculated to be ∼ 2.7 kcal/mole. A number of the internal fundamentals were observed to be split into doublets in the spectra of the solids which suggests that there are at least four molecules per unit cell in azomethane. The inversion center is apparently retained in the crystal since the rule of mutual exclusion was found to be operative.

Raman Group Frequency Correlations: Phthalate Esters

Characteristic Raman bands for phthalate esters have been established from a study of 21 compounds. The characteristic bands resulting primarily from within the ortho-phenylene group are compared with corresponding bands and assignments for ortho-dichlorobenzene. The data for phthalate esters are summarized in a correlation chart.

Raman spectra of polycrystalline isobutene- d0, isobutene- d6 and isobutene- d8

The Raman spectra of polycrystalline isobutene- d 0, isobutene- d 6 and isobutene- d 8 were recorded from 100 to 4000 cm −1. The thirty fundamental modes are assigned on the basis of group frequency correlations, isotopic shift ratios and previously reported infrared band contours. The assignment is discussed in conjunction with existing force fields. Since the low temperature Raman technique for solids is particularly useful for resolving complicated spectral patterns and for detecting weak transitions, it was possible to assign a number of the CH vibrations that were obscured in the spectra of samples in the gaseous and liquid phases.

Vibrational spectra and structure of small ring compounds–XXII

The i.r. spectra of cyclopentanol, α-cyclopentanol-d1 and β,β,β',β'-cyclopentanol-d4 in the liquid and gaseous states have been recorded from 300–4000 cm−1. The far-i.r. spectra of gaseous cyclopentanol, cyclopentanol-O-d, and cyclopentanol-d4 have been measured down to 33 cm−1. The i.r. spectra of polycrystalline cyclopentanol and cyclopentanol-O-d have been recorded from 33–300 cm−1. The Raman spectra of each species have been recorded and depolarization values measured for the normal molecule. The 42 fundamental vibrations have been assigned on the basis of group frequency correlations, isotopio shift ratios, and relative band intensities. The O-H torsional and radial modes were centered at 260 and 188 cm−1, respectively, in the spectrum of the isolated molecule but shifted to 737 and 258 cm−1 in the spectrum of the solid. Although no pseudorotational transitions were observed for gaseous cyclopentanol, a band at 122 cm−1 in the spectrum of the solid was assigned to this low frequency ring-puckering fundamental. If a frequency shift upon condensation similar to that observed for the radial mode is postulated for the pseudorotational fundamental, the 1 ← 0 transition for the isolated molecule must be lower than 50 cm−1. Such a low frequency, of course is consistent with a relatively small barrier to pseudorotation.

Vibrational spectra and structure of small ring compounds—XXI 1,1,2,2-tetrafluorocyclobutane

Abstract The infrared spectrum of gaseous and polycrystalline 1,1,2,2-tetrafluoro cyclo butane has been recorded from 33 to 4000 cm −1 and the Raman spectrum of the liquid sample has been measured from 70 to 4000 cm −1 . The vibrational data is consistent with a puckered molecule with C 2 symmetry. A vibrational assignment is proposed for the 30 fundamental modes based upon the observed band contours, relative band intensities, depolarization values and group frequency correlations. The band resulting from the ring-puckering mode was broad and centered near 90 cm −1 . Since no distinct Q -branches could be resolved, it was not possible to establish a unique potential function for this vibration; however, the data has been compared in a qualitative manner with cyclo butane and several of the substituted cyclo butanes. A correlation of this type implies that the barrier to planarity in tetrafluoro cyclo butane is 140 ± 50 cm −1 . The significance of this inversion barrier is demonstrated by a comparison of the established values for cyclo butane and 1,1-difluoro cyclo butane.

Spectra and Structure of Small Ring Compounds. XX. Fluorocyclobutane

The infrared spectrum of fluorocyclobutane vapor has been recorded from 4000–33 cm−1. The infrared spectrum of the solid at − 196°C has been recorded from 4000–200 cm−1. The Raman spectra of the liquid at room temperature and the solid have been recorded and depolarization values have been measured. The spectra have been interpreted on the basis of a puckered model ring with only one conformation which has a plane of symmetry and belongs to the Cs point group. Assignment of the 30 normal vibrations has been based on depolarization values, band contours, group-frequency correlations, and relative intensities. A series of four Q branches was observed in the far-infrared spectrum and has been interpreted as arising from the fundamental and “upper-state” transitions of the anharmonic ring-puckering vibration. The series could be fit equally well by two different double-minima potential functions, one with a relatively high barrier of 803 cm−1 and the other with a 485-cm−1 barrier. On the basis of previous microwave data and the midinfrared and Raman spectra of the solid and fluid states, the higher barrier potential is believed to more nearly describe the actual potential function governing the ring-puckering vibration; the energy difference between the two conformations was calculated to be 722 cm−1.

Vibrational spectra and structure of small ring compounds: XVIII. 3-oxetanone

The infrared spectrum of gaseous 3-oxetanone has been recorded from 33 to 4000 cm −1. The infrared and Raman spectra of the corresponding liquid sample have also been measured and the data are shown to be consistent with a planar molecule of C 2V symmetry. A vibrational assignment of the 21 fundamental modes is proposed which is based on the gas phase band contours, group frequency correlations, and depolarization values. A regular progression of Q-branches originating at 140 cm −1 has been attributed to the anharmonic ring-puckering vibration. The data were fitted with a two parameter potential function of the form V = 13.23 ( z 4+27.48 z 2), where z is the reduced ring-puckering coordinate for the planar molecule. In liquid 3-oxetanone, the ring-puckering vibration was assigned to a band at 167 cm −1 which shifted to 230 cm −1 upon crystallization. The significance of the shifts upon condensation is discussed. A short CO bond length, on the order of 1.19 ± 0.01Å, is indicated by the high frequency of the carbonyl stretching vibration.

Vibrational Spectra and Structure of Small Ring Molecules. XVI. Vibrational Analysis and Ring-Puckering Vibration of 1-Pyrazoline

The infrared spectrum of gaseous 1-pyrazoline has been measured from 33 to 4000 cm−1. The infrared and Raman spectra of the corresponding liquid sample have also been recorded. A vibrational assignment of the fundamentals is proposed which is consistent with the Cs equilibrium configuration. The assignment is based on the gas-phase band contours, the depolarization values, and group frequency correlations. A series of 13 pronounced Q branches was observed in the far-infrared spectral region, and they have been attributed to the strongly anharmonic ring-puckering vibration. Although the far-infrared frequencies could be reproduced by two different potential functions, a double minimum potential of the form V = 23.40(z4 − 4.40z2) where z is the reduced ring-puckering coordinate provides the most satisfactory interpretation of all the spectral data. The height of the inversion barrier is calculated to be 113 cm−1, and the equilibrium angle between the two dihedral planes of the puckered ring is 19.7° ± 1.0°. The results are compared with those obtained on cyclopentene.

The vibrational spectra of 2,5-dihydrothiophene

The infrared and Raman spectra of 2,5-dihydrothiophene have been measured from 4000 to 70 cm −1 . The fundamental vibrations conform to the symmetry species of the C 2 v point group in support of previous findings which state that the ring skeleton of the molecule is planar. Frequency assignments based on vapor-phase band contours, depolarization data and group-frequency correlations are presented for the twenty-seven normal vibrations.

Vibrational Spectra and Structure of Four-Membered Ring Molecules. III. Bromocyclobutane, Bromocyclobutane-d1, Bromocyclobutane-d4, and Bromocyclobutane-d5

The infrared spectra of bromocyclobutane, α-bromocyclobutane-d1, β,β,β′,β′-bromocyclobutane-d4, and α,β,β,β′,β′-bromocyclobutane-d5 in both the liquid and vapor phases have been recorded from 4000 to 250 cm−1. The far-infrared spectra of bromocyclobutane and β,β,β′,β′-bromocyclobutane-d4 have been recorded to 33 cm−1. The Raman spectra of the four isotopic species of bromocyclobutane in the liquid state have also been recorded and depolarization values have been measured; the carbon—bromine stretching region of bromocyclobutane-d1 has been scanned at temperatures varying from 35° to −83°C. The spectra have been interpreted by considering the molecules as having one plane of symmetry and belonging to point group Cs. Frequency assignments which are based on group-frequency correlations, isotopic shifts, depolarization values, band contours, and relative intensities where applicable are given for the normal vibrations. The assignments are tabulated and discussed according to symmetry species. The regimens of the product and noncrossing rules have been followed. Support for the existence of ``equatorial'' and ``planar'' conformers is presented and the nature of the potential function governing the ring-puckering vibration for bromocyclobutane is discussed.

Factors affecting the out-of-plane hydrogen deformation frequencies in olefins and their derivatives

The various infrared absorption group frequency correlations for out-of-plane hydrogen deformations of substituted ethylenes are reviewed. It is shown that the CH2 wag motion of vinyl and vinylidene type substituted ethylenes behave in an analogous manner, and the twist motion in vinyl and trans-type substituted ethylenes also behave in an analogous manner. The CH2 wag frequency appears to be related to the resonance effects of substituent groups, while the twist frequency appears to be related to the inductive effects of substituent groups.