Stephen Bell

Locating transition states

A previously reported method for locating saddle points on multidimensional surfaces by a combination of aspects of the conjugate gradients method and the quasi‐Newton minimization method is now described with modifications to make it more robust. A number of model potentials from the literature and some ab initio molecular potential energy surfaces are used for test and comparison purposes.

A new efficient method for locating saddle points

Abstract A direct method is described for locating saddle points on multidimensional surfaces. It combines aspects of the conjugate gradient method and the quasi-Newton minimization method.

An emission system of CS in the vacuum and near ultraviolet

Abstract A new emission system of CS has been observed in the vacuum and near ultraviolet produced by a microwave discharge in carbon disulphide. The system arises from the transition A ′ 1 Σ + -X 1 Σ + . Twelve vibrational levels of the X state and nine levels of the A ′ state have been observed and rotational constants obtained for most of them. The following vibrational and rotational constants have been derived for the A ′ state: ω e =462.4, ω e χ e =7.46, ω e Y e =−0.108, ω e Z e =0.0377cm − 1, β e =0.5114, α e =0.0109, γ e =0.00112, δ e =0.000208cm − 1. From these constants we derive r e ′ = 1.944 ± 0.001A, T 0 = 56093cm −1 and D 0 ′ = 3270cm −1 , and hence an accurate value is found for the ground state dissociation energy, D 0 ″ = 59320 ± 100cm −1 .

The π∗ ← n transition of formic acid

Abstract New sharp bands of formic acid have been observed in the near ultraviolet at the long wave-length end of the previously observed diffuse band system (2250–2500 A) by considerably extending the absorption path length. Both the diffuse and sharp bands belong to the same vibrational system which is assigned to the π ∗ ← n electronic transition in the carbonyl group. Extensive progressions are observed in the carbonyl stretching frequency which is greatly reduced in the excited state (fundamental ν3′ ≈ 1080 cm−1) and many intervals of about 400 cm−1 are assigned to the OCO bending frequency ν7′. A band contour analysis of the 2593 A band shows that the molecule is nonplanar in the excited state because of the magnitude and sign of the inertial defect. From this analysis, the rotational constants for the excited state are S ‘ =1.8755, B ‘ 0.4042, C ‘ =0.3378 cm −1 By the plausible assumption that the important changes in the molecule are in the C=0 bond length, the OCO angle, and the nonplanarity due to the formyl hydrogen, the following excited state parameters are derived.rC=0 = 1.407A.The changes in formic acid are closely analogous to the changes in formyl fluoride as a result of the π ∗ ← n transition.

Structure and potential energy functions for acetaldehyde: Ab initio calculations of X̃1A′, Ã1A″, and B̃1A′ states

Abstract The geometrical structure of the ground state of acetaldehyde obtained by optimization of ab initio SCF energies is compared with a number of experimental structures derived from microwave spectra. The optimum geometries of acetaldehyde in its two lowest singlet excited states, A 1 A″ and B 1 A′ , are also determined. A force constant matrix found for each of the X 1 A′ and B 1 A′ states is used to calculate the vibrational wavenumbers of the four isotopic species, CH 3 CHO, CH 3 CDO, CD 3 CHO, and CD 3 CDO, and thus to check assignments of observed wavenumbers from infrared spectra. The potential energy function for internal rotation of the methyl group is also studied in each of the three states and for inversion in the A state. Reasonable agreement is obtained between ab initio barrier heights and values derived from microwave, far infrared, and ultraviolet spectra.

Electronic structure and spectrum of cyanogen

Abstract Ab initio RHF calculations have been made with a double-zeta basis of contracted gaussian functions for the ground state of C 2 N 2 .40 excited states of different configurations and symmetrics, and four states of C 2 N + 2 . The assignments of known UV absorption and PE spectra are considered in the light of calculated excitation energies.

Torsional transitions and barrier to internal rotation of 1-butyne

Abstract The far infrared spectrum of 1-butyne (ethyl acetylene) has been recorded in the gas phase from 370 to 40 cm −1 with a resolution of 0.1 cm −1 . The skeletal bending fundamental transitions ν 23 ( a ″) and ν 15 ( a ′) give strong bands at 343.6 and 196.5 cm −1 , respectively, showing detailed rotational structure. Transitions of the torsional mode, ν 24 ( a ″), are assigned to prominent Q spikes superimposed on the ν 15 band at 212.8 and 201.5 cm −1 . From these data the coefficients of the periodic potential function are calculated to be 1060 and −30 cm −1 for the V 3 and V 6 terms, respectively.

Internal rotation in acetone

Abstract The geometrical structure of the ground state of acetone obtained by ab initio SCF energies is compared with experimental structures mainly from microwave spectroscopy. The potential function for the internal rotational motions in acetone obtained from SCF energies is compared with experimental. It is shown that the top-top coupling constants are not negligible in acetone, and that at least three constants are required to give realistic potential functions for internal rotation in dimethyl molecules. Torsional potential constants for acetone of V3 = 452, V33 = 174, V′33 = −171, and V6 = 1 cm−1 are calculated with the DZ basis.

Infrared and Raman spectra and ab initio calculations for 2-pentyne

Abstract The infrared spectra (3500–50 cm −1 ) of the gas and solid and the Raman spectrum (3500–30 cm −1 ) of solid 2-pentyne, CH 3 CH 2 CCCH 3 , have been recorded. Additionally, the infrared spectrum (3500–400 cm −1 ) of a xenon solution has been recorded. A complete vibrational assignment is proposed based on infrared band contours, relative intensities, depolarization values, and group frequencies. The assignment is supported by normal coordinate calculations utilizing ab initio force constants. The internal rotational barrier for the CH 3 rotor of the ethyl group was determined to be 1285 cm −1 from the torsional transitions whereas that for the CH 3 rotor attached to the carbon of the triple bond has nearly free rotation. Complete equilibrium geometries have been determined employing several basis sets at the levels of restricted Hartree–Fock (HF), and/or with full electron correlation by the perturbation method to second order (MP2) as well as with a hybrid density functional theory (B3LYP). The results are discussed and compared to those obtained for some similar molecules.

Infrared and Raman spectra, conformational stability, ab initio calculations of structure and vibrational assignment of butyronitrile

The infrared spectra (3500–50 cm−1) of the gas and solid and the Raman spectra (3500–50 cm−1) of the liquid and solid were recorded for butyronitrile (propyl cyanide), CH3CH2CH2CN. Both the anti (methyl group trans to the cyanide group) and gauche conformers were identified in the fluid phases. Variable temperature studies of the infrared spectrum (3500–400 cm−1) of the sample dissolved in liquid xenon (−60 to − 100°C) were recorded. Utilizing nine sets of conformer pairs, the enthalpy difference was determined as 40 ± 3 cm−1 (0.48 ± 0.04 kJ mol−1) with the gauche conformer the more stable form. At ambient temperature there is approximately 30% of the anti conformer present in the vapor state. Either the anti or gauche conformers could be obtained in the crystalline state depending on whether the solid was obtained from the liquid or from the gas sprayed on a cold substrate. A complete vibrational assignment is proposed for both conformers based on infrared contours, relative intensities, depolarization values and group frequencies which are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6–31G(d) calculations. Complete equilibrium geometries were determined for both rotamers by ab initio calculations employing the 6–31G(d), 6–311 + G(d,p) and 6–311 + G(2df,2pd) basis sets at the level of Moller–Plesset with full electron correlation by the perturbation method to the second order (MP2). From all of these calculations the gauche conformer is predicted to be the more stable form. The r0 adjusted structural parameters were obtained from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.