Lionel Goodman

Hyperconjugation not steric repulsion leads to the staggered structure of ethane

Many molecules can rotate internally around one or more of their bonds so that during a full 360° rotation, they will change between unstable and relatively stable conformations. Ethane is the textbook example of a molecule exhibiting such behaviour: as one of its two methyl (CH3) groups rotates once around the central carbon–carbon bond, the molecule will alternate three times between an unstable eclipsed conformation and the preferred staggered conformation. This structural preference is usually attributed to steric effects; that is, while ethane rotates towards an eclipsed structure, the electrons in C–H bonds on the different C atoms are drawing closer to each other and therefore experience increased repulsion, introducing a rotation barrier that destabilizes the eclipsed structure. Stabilization of the staggered structure through rotation-induced weakening of the central C–C bond and hyperconjugation has been considered to be involved, but evaluation of the contributions of these effects to ethanes internal rotation barrier and conformational preference remains difficult. Here we report a series of ethane structure optimizations, where successive removal of different interactions indicates that ethanes staggered conformation is the result of preferential stabilization through hyperconjugation. Removal of hyperconjugation interactions yields the eclipsed structure as the preferred conformation, whereas repulsive forces, either present or absent, have no influence on the preference for a staggered conformation.

The benzene ground state potential surface. I. Fundamental frequencies for the planar vibrations

The accuracy of vapor phase vibrational data has been improved for all 12 deuterium‐labeled benzenes and for 13C12C5H6 and 13C6H6. Many vapor phase fundamental frequencies are observed for the first time. Precise isotopic frequency/splitting patterns for ν1, ν18, and ν19 have been obtained. Isotope induced harmonic mode mixing matrices are given for all 14 labeled benzenes and used to provide detailed description of the fundamental bands observed in the spectra. These descriptions provide numerous reassignments for the fundamental bands, particularily in low symmetry deuterium benzenes. The matrices show that some skeletal modes, such as ν1, gain CH stretching character as a result of deuterium labeling, providing a rationalization for the increased anharmonicity observed in recent jet experiments for C6D6. In addition, a reassessment of Fermi resonance gives 3072.3 cm−1 for the unperturbed frequency (correction +24 cm−1) for the e1u mode ν20 in C6H6 refining the CH local mode anharmonic constant, 2xii, t...

The benzene ground state potential surface. II. Harmonic force field for the planar vibrations

A complete harmonic force field in terms of nonredundant coordinates has been generated from experimental frequencies for D6h, D3h, and D2h isotopically labeled benzenes and degenerate mode Coriolis constants predicting broken symmetry labeled benzene frequencies to ±0.1% and Coriolis constants to ±0.01 units, on the average. Exact solutions have been obtained for the six E1u force constants from D6h symmetry frequency data with the inclusion of 13C6H6 information. Some modes (e.g., the e2g mode ν8, in Wilson notation) are significantly altered from previous experimental force field predictions, rationalizing unclearly understood vibronic features of phosphorescence and two‐photon spectra. A conundrum regarding the e1u Coriolis constant for ν18 (Wilson notation) is identified: no harmonic force field is capable of predicting the reported experimental magnitudes for this constant for both C6H6 and C6D6. The Pulay et al. scaled ab initio force field is in qualitative agreement with the experimental field fo...

Assignment of out-of-plane vibrational modes in benzaldehyde

Abstract Infrared spectra, Raman spectra and valence force field calculations of normal coordinates are used to assign the benzaldehyde out-of-plane modes. Definite assignments from the solution spectra are 133 cm −1 aldehyde torsion, 232 cm −1 CHO out-of-plane wag, 449 cm −1 ring bending-twisting motion probably corresponding largely to ν 16 b 688 and 746 cm −1 out-of-plane phenyl modes corresponding to mixed ν 4 and ν 11 in benzene and 1010 cm −1 as the out-of-plane aldehyde hydrogen wag. Less definite assignments are 410 cm −1 as an out-of-plane skeletal distortion of the benzene ring (i.e., largely ν 16 a ) and 925 cm −1 as ν 17 b . In-plane modes incidentally obtained are 221 cm −1 φ-CHO bend, 1394 cm −1 CH aldehyde bend, and 1707 cm −1 CO stretch. Three bands at 442, 650, and 834 cm −1 which are firmly assigned as in-plane vibrations arise from strongly mixed aldehyde and benzene modes. The remaining in-plane modes are correlated with their benzene analogues.

Multiphoton ionization photoelectron spectroscopy of phenol: Vibrational frequencies and harmonic force field for the 2B1 cation

A molecular beam of phenol, cooled by a supersonic expansion, is crossed at right angles by the output of a pulsed frequency‐doubled dye laser, causing 1+1 resonance enhanced multiphoton ionization. The kinetic energy of the resulting photoelectrons is determined as a function of laser wavelength with time‐of‐flight analysis, permitting the assignment of 11 vibrational frequencies for the 2B1 phenol‐h6 cation and ten vibrational frequencies for phenol‐d5. Of these, all but the lowest frequency one in each case are in‐plane vibrations of which phenol has a total of 19. An approximate harmonic force field for the in‐plane modes of the phenol cation is derived along with its associated frequencies and mode forms. This in turn facilitates the vibrational analysis. Analogous force field calculations have been carried out on the ground (1A1) and first excited (1B2) states of the neutral parent, permitting conclusions to be reached concerning bonding changes upon removal of an electron from the phenol electron s...

Multiphoton resonance ionization bands in I2

Multiphoton resonance ionization (MPRI) bands of I2 vapor have been examined in the 360–600 nm region. A band system with T0 =48 426±4 cm−1 and ωe=254±1 cm−1 is assigned to a two‐photon resonant intermediate state involved in either three‐ or four‐photon ionization. Polarization measurements establish that the transition is either 1g or 2g←0g. Comparison to the Dalby system (2d), 5146 cm−1 to higher energy allows a more detailed interpretation of the excited states of both systems: 2g (2Π3/2g) nsσg and 1g (2Π1/2g) nsσg, respectively. Excitation into the visible B←X absorption band between 500 and 600 nm leads to a dense set of MPRI lines. An intense system in this region with T0 =35 762±4 cm−1 and ωe=104±1 cm−1 is assigned to a two‐photon resonant intermediate state with polarization measurements establishing it as 0g. It is further interpreted as arising from the two‐electron promotion 0g→σu, πu→σu. A striking feature of the MPRI spectra of I2 is the near absence of lines corresponding to the normal opti...

Flexing analysis of ethane internal rotation energetics

A flexing analysis of the ethane barrier energy in terms of structural (ΔEstruct), steric exchange (ΔEsteric), and hyperconjugative charge-transfer (ΔEdeloc) energy contributions has been carried out using natural bond orbitals. No evidence is found for the view that the ethane staggered equilibrium geometry or the C–C bond expansion that accompanies rotation results from steric exchange repulsion interactions. The analysis shows that ΔEstruct and ΔEdeloc have very different stereoelectronic dependencies, but that the ΔEsteric and ΔEdeloc dependencies are antagonistic. All of their contributions are strongly affected by the C–C bond expansion, with the result that the barrier mechanism cannot be understood without taking into account their different relaxation dependencies. Neglect of C–C expansion leaves the charge-transfer interactions paramount by subduing the steric and structural contributions. These interactions are found to be an important determinant for the expansion. The strong expansion depende...

TRIPLET STATE OF BENZALDEHYDE.

The phosphorescence spectra of benzaldehyde‐h6, −1d1, −4d1, −3, 5d2 and −d6 have been studied optically in close packed matrices (acetophenone and methyl benzoate) and relaxed matrices (methylcyclohexane and perfluoro‐n‐hexane) at 4.2°K. All the spectra were sharp and could be analyzed vibrationally. In the close packed matrices, of the 11 possible out‐of‐plane (a″) fundamentals, nine are observed in Herzberg—Teller borrowing of intensity. The strongest modes are the aldehyde‐H wag, the CHO torsion and CHO wag, and their intensities were found to be stronger than those of any totally symmetrical bands including the origin band. 2 and 3 quanta of aldehyde‐H‐wag and CHO‐torsional modes are found with intensity alternation. In addition to this intensity alternation, the lack of long sequences of any nontotally symmetric mode and the analysis of the Franck—Condon envelope show that the triplet benzaldehyde does not deviate far from the ground state geometry. We conclude it is coplanar or nearly so. Deuteratio...

Vibronic mechanisms in the two‐photon spectrum of benzene

Intensity measurements have been made on the single quantum bands of the active modes in the two‐photon 1B2u←1A1g spectra of C6H6 and its isotopic homologs C6H5D, o‐C6H4D2, C6HD5, p‐C6H4D2, p‐C6H2D4, s‐C6H3D3, and C6D6. ν14 (b2u) and ν18 (e1u) (D6h symmetry) are found to be active in all the isotopic benzenes, and ν12 (b1u) in all except C6H6, s‐C6H3D3, and C6D6 where it is identity forbidden, and p‐C6H4D2 where it is weak. The results show that the band strength of 1410 is insensitive to deuterium substitution (confirming Duschinsky rotation of this mode), while 1210 and 1810 are strongly sensitive to the number and orientation of the deuterium atoms. INDO/S calculations of transition tensors at displaced nuclear coordinates, with and without doubly excited configurations, indicate that the ground state vibronic mechanisms usually neglected in one‐photon spectra are important in the benzene two‐photon spectra. The activity of ν14 can be attributed to vibronic perturbation of the final B2u state by the gr...

Allowed and Forbidden Character in the 3715‐Å π* ←n System of Benzaldehyde

The 3715‐A system of benzaldehyde‐h6, ‐d1, and ‐d6 has been observed in absorption in the vapor phase. In spite of some diffuseness two band types have been distinguished by their characteristic rotational contours. Bands showing one main peak in the contour belong to the electronically allowed part of the system, assigned as 1A″(nπ*)←1A′, and those showing two main peaks split by about 9 cm−1 belong to the electronically forbidden part of the system involving activity of nontotally symmetric (a″) vibrations. Assignment of the wavenumbers of the three most strongly active a″ vibrations to approximate normal coordinates is certain in the case of the CHO‐torsional vibration but less certain in the cases of the out‐of‐plane substituent vibration and an out‐of‐plane ring vibration. Qualitative estimates of intensity borrowed through activity of these three vibrations have been made using undistorted and distorted molecule wavefunctions of the Pople–Santry–Segal type. The calculated n‐electron density is highl...