T.L. Smithson

The vapor phase far‐infrared spectra of 1H‐indene, benzo[b]furan, 1,3,2‐benzodioxaborole, and indole: Evidence for planar skeletons and kinetic anharmonicity in the out‐of‐plane ring deformations

The far‐infrared spectra of the vapors of 1H‐indene, benzo[b]furan, 1, 3, 2‐benzodioxaborole (catechol borane), and indole are reported in the region of 100–500 cm−1. The spectra of the first three compounds exhibit series of C‐type Q branches for the three out‐of‐plane ring deformations, establishing planar skeletal configurations and indicating that the aromatic skeletons are not truly rigid. The lower of these two sequences are assigned as interlocking transitions arising from the deformation of the five‐membered ring and the butterfly mode, and are analyzed by means of a two‐dimensional Hamiltonian which includes for each mode a quadratic potential constant and a quadratic dependence of the reduced mass on each displacement coordinate, as well as a potential constant coupling the two modes.

1,3-benzodioxole: far-infrared spectrum 50–500 cm−1

Abstract The far-infrared spectrum of the vapour of 1,3-benzodioxole in the region of 50–500 cm −1 is reported and interpreted in terms of combination and interstate transitions involving four skeletal deformations. Analysis of the out-of-plane deformation sequence of the five membered ring yields a simple quadratic-quartic potential function with a positive quadratic coefficient suggesting a planar structure for the molecule.

The ring puckering vibration of trimethylene sulfoxide: Far-infrared spectrum from 50 to 675 cm−1

Abstract A number of absorptions between 100 and 300 cm −1 in the infrared spectrum of trimethylene sulfoxide vapor have been observed and assigned to Δ v p = 1 and 2 transitions in the ring puckering mode. With the aid of a computer program providing for a potential function essentially of the form V ( x ) = V 2 x 2 + V 3 x 3 + V 4 x 4 ( x the dimensioned puckering coordinate), the barrier to inversion between the stable equatorial and a possible axial conformer is established to be 1205 cm −1 with an uncertainty of ±11 cm −1 at the worst or ±1 cm −1 at best, depending upon the assignment of several very weak absorptions. The axial well depth, depending more critically upon these assignments, is no greater than 150 cm −1 and may be as low as 5 cm −1 .

The ring-puckering vibration of methyl-substituted oxetans and thietans: far-infrared spectra from 50 to 500 cm−1

Abstract The vapour-phase infrared spectra in the range 50–500 cm −1 are reported for 2- and 3-methyl oxetan (2- and 3-MO), 2-methyl thietan (2-MT), 2,2-dimethyl oxetan (2,2-DMO) and 3,3-dimethyl thietan (3,3-DMT). The absorptions are interpreted as methyl deformation fundamentals and transitions within the ring-puckering manifold. Particular emphasis is placed on the latter, for which band sequences are analyzed with a general puckering potential function, V ( q ) = V ′ 2 q 2 + V ′ 3 q 3 + V ′ 4 q 4 , q being the dimensionless puckering normal coordinate. For the gem -dimethyl compounds for which V ′ 3 = 0, the barrier to ring inversion increases upon methyl substitution to 51.1 cm −1 for 2,2-DMO and 300.0 cm −1 for 3,3-DMT as compared to the parent molecules. The monomethyl compounds exist as stable equatorial and axial conformers at room temperature, the energy differences being 38.1, 43.8 and 117.6 cm −1 , and the inversion barriers 107.5, 98.7 and 341.1 cm −1 , respectively, for 2-MO, 3-MO and 2-MT. The trends in the potential function constants are discussed.

The out-of-plane ring deformations of 1,3- and 1,4-dioxene and 3,6-dihydro-2h-pyran: comparison of a series of cyclohexenes

Abstract The far-IR spectra (100–600 cm − ) of 1,3-dioxene and 3,6-dihydro-2H-pyran in the vapour phase are analyzed for the out-of-plane ring deformations. The fundamentals and a number of combination sequences are assigned. The spectrum of 1,4-dioxene is reexamined. The positions of these deformations in the three compounds are compared with those of cyclohexene and 2,3-dihydropyran. The twisting transitions are analyzed with a one-dimensional quadratic-quartic potential function. The barrier heights so derived, thought to represent approximate values for the barrier to planarity, decrease in the order 1,4-dioxene > 2,3-dihydropyran > 3,6-dihydro-2H-pyran > 1,3-dioxene.

The far-infrared spectrum of tetrahydrothiophene: The bend/twist vibrations and associated barriers

Abstract The far-infrared spectrum of tetrahydrothiophene is reinvestigated with a resolution of 0.12 cm−1 in the region of 50–350 cm−1. In addition to the bend transitions (νb) below 120 cm−1 reported previously, a number of sequences revealed by the improved resolution are observed for the first time and assigned to transitions of 2νb, of the twist (νt), and of difference combinations (νt - νb). Simple one-dimensional modeling of the twist sequence, which is derived from a self-consistent bend-twist energy level diagram, with a quadratic-quartic Hamiltonian suggests a barrier to planarity on the order of 4250 cm−1. A two-dimensional potential function in the dimensionless coordinates is found to be V(qb, qt) = −249.6qb2 + 4.48qb4 − 215.5qt2 + 2.73qt4 + 7.00qb2qt2.

The ring puckering vibration of 3,3-dimethyloxetan

Abstract The far-infrared vapor phase spectrum of 3,3-dimethyloxetan, consisting of a series of overlapping B contours around 90 cm −1 and a sequence of A contours upward of 130 cm −1 , has been assigned in terms of Δ v p = 1 and 2 transitions in the ring puckering mode. The constants in the potential function, V ( Z ) = A ( Z 4 + BZ 2 ), Z the reduced puckering coordinate, are calculated to be A = 19.24 cm −1 and B = −3.106, and the inversion barrier 46.4 cm −1 .

The far-infrared spectra and inversion barriers of some cyclohexenones

Abstract The far-infrared absorptions resulting from the inversion of the six-membered ring through the molecular plane are reported for 3-cyclohexen-1-one (3CHO), 3-methyl-2-cyclohexen-1-one (3M2CHO), and 4-methyl-3-cyclohexen-1-one (4M3CHO). The corresponding spectrum of 2-cyclohexen-1-one (2CHO) with improved resolution is included for comparison. The methyl torsion absorptions of 4M3CHO are also analyzed. The barriers to planarity are calculated with a one-dimensional Hamiltonian using a quadratic-quartic potential function, giving barrier heights of 3642, 4567, 463.0, and 431.2 cm−1, respectively, for 2CHO, 3M2CHO, 3CHO, and 4M3CHO.

Ring puckering combinations with the totally symmetric ring stretching vibration in some deuterated analogs of trimethylene oxide as resolved by cars

Abstract The complex Raman lines of the totally symmetric ring breathing modes of α-d 2 , β-d 2 and trimethylene oxide are resolved by Coherent Antistokes Raman Scattering (CARS) into sequences of combination transitions with the ring puckering vibration. Specific assignment of the sequence is possible for the β-d 2 compound with the help of corresponding infrared sum bands. The implications for the effective puckering potential function in the excited state of the second mode, and the suitability of the CARS methods for problems of this kind is discussed.

Anharmonic skeletal deformation in bicyclo[2.2.1]hept-2-ene and 7-oxabicyclo[2.2.1]heptane

Abstract The infrared spectra in the region 50–600 cm −1 are reported for bicyclo[2.2.1]hept-2-ene (norbornene) and 7-oxabicyclo[2.2.1]heptane (7-oxanorbornane) in the vapour phase. All absorptions in this region show evidence of at least one low-energy anharmonic vibration. One sequence of Q branches in each of the two compounds is assigned to the first overtone transitions of skeletal deformations resembling a twisting of the six-membered rings. The transitions can be reproduced successfully by a one-dimensional Hamiltonian with positive quadratic and quartic terms. These observationsagrees with the structure derived from the microwave spectra and from electron diffraction for both compounds.