Herbert L. Strauss

Trimethylene Oxide. III. Far‐Infrared Spectrum and Double‐Minimum Vibration

The 0–1 transition of the out‐of‐plane bending vibration in trimethylene oxide has been observed at 53.4 cm−1. Previously reported bands by Lord and his co‐workers [J. Chem. Phys. 33, 294 (1960)] at 89.8, 105.2, 118.3, 128.9, 139.0, 147.6, 154.9, and 161.8 cm−1 have also been confirmed. In addition, six higher members of this series as well as three members of the Δv = 3 series have been observed. These transitions are readily interpreted in terms of the hot‐band series of a slightly perturbed quartic oscillator. A double‐minimum potential function of the form az4—bz2 predicted all the observed transitions to within experimental error (0.5 cm−1 or better). The central barrier in the potential‐energy function, while low, is finite and has a height of 15.3±0.5 cm−1. It had previously been concluded, from the variation of the rotational constants with vibrational state obtained from microwave spectroscopy that the potential energy contained a small barrier of the order of zero‐point energy at the planar conf...

The distribution of conformational disorder in the high‐temperature phases of the crystalline n‐alkanes

The distributions of conformational defects that exist in the high‐temperature phase II (also referred to as the hexagonal or rotator phase) of the crystalline n‐alkanes C21 and C29 have been measured by an infrared CD2‐substitution technique and have been accounted for in terms of a lattice model that provides freedom for longitudinal displacement of the chains. The defects consist almost entirely of gtg’ kinks distributed nonuniformly along the chain. The uneven distribution is indicated in the variation in the concentration of gauche bonds measured at various sites along the chain. The highest concentration is at the chain ends, and the concentrations at interior sites decrease exponentially in going toward the middle. To explain the distribution we used a modification of a lattice model that had been successfully applied to the lipid bilayer. Comparison of observed distributions with those computed from the model indicates that the factors that determine the shape of the distribution are quite differe...

Phase Transitions and Nonplanar Conformers in Crystalline n-Alkanes

Crystals of n-alkanes show a remarkable series of solid-solid phase transitions. In the odd n-alkanes C25, C27, and C29 a previously unknown transition is found by both calorimetry and infrared spectroscopy. The ubiquitous presence of nonplanar conformations of the chains is shown by infrared spectroscopy. The nonplanar conformers constitute approximately half the molecules in the highest temperature solid phase of C29.

Far‐Infrared Spectra of Trimethylene Sulfide and Cyclobutanone

The far‐infrared spectra of trimethylene sulfide and cyclobutanone have been investigated. The positions of the trimethylene sulfide bands observed are well fitted by the potential function derived from the microwave studies of Harris et al. However, some of the band shapes cannot be explained in detail although the anomalies are probably due to Coriolis terms in the Hamiltonian. The positions of the lower frequency bands of cyclobutanone can be fit approximately by a quartic—quadratic potential with a small barrier. Although this potential function fits the rotational constants measured by Sharpen et al., it does not predict the positions of the higher frequency bands correctly. Furthermore the band shapes cannot be explained on the basis of the simple potential function, and it is suggested that the low‐frequency out‐of‐plane vibration of cyclobutanone is perturbed by a higher frequency vibration.

Isolated C–H stretching vibrations of n‐alkanes: Assignments and relation to structure

The conformational/configurational dependence of the frequencies of the deuterium‐isolated C–H stretching modes of the gas‐phase alkanes C1, C2, n‐C3, n‐C4(t and g), n‐C5(tt and gt), cyclo‐C6, iso‐C4, and neo‐C5 are reported. Most of the isolated C–H frequencies were obtained from Raman spectra of specifically and randomly protonated deuterohydrocarbons. An extraordinarily precise correlation is found between the observed isolated C–H frequencies and the corresponding ab initio calculated C–H bond lengths. In the case of the n‐alkanes, the observed C–H frequencies tend to fall in clusters that are regularly spaced with an average separation of about 14.5±1 cm−1. The clustering occurs because the isolated C–H stretching frequencies are determined by the structure of the n‐alkane in the immediate vicinity of the C–H bond. The relation between frequency and local structure can be expressed in a simple way and used to predict the effect of conformational change.

The motional collapse of the methyl C–H stretching vibration bands

The asymmetric CH stretching bands of the methyl groups of a number of alkane systems have been examined as a function of temperature. At low temperatures, two bands are seen for a CH3 group. These bands collapse into one as the temperature is raised. Examination of crystals of alkanes with both odd and even numbers of carbon atoms, alkanes in urea clathrates, and deuterium substituted alkanes allows the separation of intramolecular and intermolecular contributions to the band splittings and widths. Model Hamiltonians that express the stretching energy levels as a function of both the methyl torsional coordinates and the external coordinates of the alkanes are set up. These are used in the Redfield equations to derive expressions for the splittings and widths in terms of parameters which give the coupling of the various types of motion. The splittings are found to be proportional to the average of cos 5ϑ, where ϑ is the torsional angle, and a contribution to the width is found to be due to fluctuations in...

Far‐Infrared Spectrum of Tetrahydrofuran: Spectroscopic Evidence for Pseudorotation

The infrared spectra of tetrahydrofuran and tetrahydrofuran‐d8 are reported between the limits 10 and 300 cm−1. The rather complex spectrum obtained for this molecule is interpreted in terms of the theory of pseudorotation of a puckered ring. The spectroscopically derived value of the parameter mq02 is (8.56±0.13)×10−40 g·cm2 for the ground, v=0, radial state and (8.48±0.15)×10−40 g·cm2 for the excited, v=1, radial state.

Spectroscopic Evidence for Pseudorotation. II. The Far‐Infrared Spectra of Tetrahydrofuran and 1,3‐Dioxolane

The absorption spectra of tetrahydrofuran and of 1,3‐dioxolane at about 0.3‐cm−1 resolution are reported in the region of 20–370 cm−1. It is found that dioxolane pseudorotates, and the earlier evidence for the pseudorotation of tetrahydrofuran is confirmed. Both compounds have barriers to pseudorotation of about 50 cm−1. Pseudorotation constants in both the ground and in the first excited radial states are obtained. The spectra show the effects of a considerable number of complex rotation–vibration interactions, which can be understood qualitatively by comparison with the spectra of four‐membered‐ring compounds. The band shapes are explained semiquantitatively using a number of Coriolis terms in the effective Hamiltonian.

Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. I. The Raman excitation profile of I2 in n‐hexane

The absolute Raman excitation profiles (REPs) of the fundamental and first two overtones of I2 dissolved in n‐hexane were determined with excitation frequencies from 15 000 to 22 000 cm−1. Calculations for both the absorption spectrum and the REPs were performed and the results compared to the experimental results. Good agreement was found using gas phase potentials for the A, B, and B‘ states and a ground state potential modified to reproduce the observed anharmonicity of the I2 vibrations in n‐hexane. The homogeneous linewidth in the excited electronic states is Γ=15–20 cm−1 (HWHM), which corresponds to a T2 of ≊0.3 ps. The spectra have an inhomogeneous component of 400 cm−1 (HWHM). Separate calculations demonstrate the effects of the excited electronic states individually and demonstrate the importance of the interference terms in the REP. Calculations also demonstrate the sensitivity of the depolarization ratio to the contributions of the various excited electronic states. The index of refraction corr...

Can the Bloch equations describe the vibrational spectra of a reacting molecule

The vibrational spectra of molecules that are rapidly interconverted among distinct species by very fast motions, for example, during ordinary chemical reactions or during the rotation of a molecule between different sites in a solid, are considered. The question of the title is addressed in a number of distinct stages. First, the spectra predicted by Bloch equations with the inclusion of exchange terms are derived. The results differ from those familiar from magnetic resonance spectroscopy, since the vibrational transition moment can have a different magnitude and orientation in each site. Next, the question of whether a reaction can be fast enough on the time scale required for the simple vibrational Bloch equations to be valid is addressed, and it is concluded that this is unlikely. The observed spectrum may be fit with the result of the Bloch equation analysis (as has been done often in the past), but we conclude that the rate of the reaction cannot be simply extracted from the parameters used in this...