V. P. Novikov

Applications of spline functions in programs for gas phase electron diffraction analysis

Abstract The applications of spline functions in programs for gas phase electron diffraction analysis are reported. All manual operations involved in the initial background determination and further corrections to it are eliminated by using a spline approximation. The computer programs are described in some detail.

NIST-JANAF Thermochemical Tables. II. Three Molecules Related to Atmospheric Chemistry: HNO3, H2SO4, and H2O2

The structural, spectroscopic, and thermochemical properties of three polyatomic molecules with internal rotation—HNO3(g), H2SO4(g), and H2O2(g)—have been reviewed. Three revised ideal gas thermodynamic tables result from this critical examination. The revisions involved the consideration of new spectroscopic information and the use of theoretical results to model the internal rotation in the H2SO4 molecule. Compared to previous calculations, the entropies at 298.15 K are unchanged for HNO3 and H2O2, but the high temperature values (T>4000 K) are significantly different. As for H2SO4, its thermodynamic functions differ significantly from values calculated earlier.

NIST-JANAF Thermochemical Tables. I. Ten Organic Molecules Related to Atmospheric Chemistry

The structural, spectroscopic, and thermodynamic properties of 10 gas phase organic molecules related to atmospheric chemistry, including three peroxides and four carboxylic acids, are reviewed. The calculation of the thermochemical tables involved the critical evaluation of new spectroscopic data, enthalpy of formation determinations, and the use of recent internal rotation data. Since insufficient information to characterize all 10 molecules exists, estimation schemes were used to provide the missing experimental and theoretical data.

Density functional theory study of conformations, barriers to internal rotations and torsional potentials of polychlorinated biphenyls

Abstract Torsional barriers, potential energy curves, structural parameters and vibrational frequencies were calculated for 119 polychlorinated biphenyls (PCBs) at the B3LYP/6-31G(d,p) density functional theory level. From these calculations, the molecular parameters of remaining 90 PCB congeners can be estimated with reasonable accuracy. The conformations and torsional barriers of PCBs are mainly determined by the number of ortho chlorine atoms, while the meta chlorine atoms have a small influence on the torsional angles and barriers. All 209 PCBs were sorted into 18 groups depending on the number of ortho and adjacent meta substituents. In each group the internal rotation behavior of PCB congeners is very similar. For PCBs with 2–4 ortho chlorine atoms, the calculated barrier heights are 10–80 kJ/mol higher than those determined earlier by semiempirical AM1 method.

Ideal gas thermodynamic properties of biphenyl

Ideal gas thermodynamic properties for biphenyl have been calculated by statistical thermodynamics method on the basis of available experimental data and the results of recent high-level quantum mechanical calculations. More precise information about the potential barriers to internal rotation and the lowest vibrational frequencies made it possible to achieve the best agreement with calorimetric results in comparison with statistical calculations published earlier. The thermodynamic functions have been also estimated using molecular structure, vibrational frequencies, and potential barrier heights of biphenyl calculated by B3LYP/cc-pVTZ density functional method.

Molecular structures of acetylene derivatives of tin: Part II. Gas phase electron diffraction study ofbis(trimethylstannyl)acetylene, Me3SnCCSnMe3

Abstract A gas phase electron diffraction study of bis(trimethylstannyl)acetylene is reported. The ra structure refines to the following parameters (bond lengths in nm, valence angles in degrees): The numbers in parentheses are three times the standard deviation values. The difference between the bond lengths in the bis(trimethyl-stannyl) acetylene molecule is less significant than with trimethylstannylacetylene. The observed departure from linearity in the Sn-CC-Sn fragment is seemingly due to the shrinkage effect.

Molecular structures of acetylene derivatives of tin: Part III. Gas phase electron diffraction study of tetrakis(trifluoropropynyl)-tin, Sn(CC-CF3)4

Abstract A gas phase electron diffraction study of tetrakis(trifluoropropynyl)tin is reported. The model, based on T d symmetry for the carbon—tin skeleton and C 3v symmetry for the CF 3 groups, refines to the following parameters (bond lengths, r a , in nm; valence angles in degrees): Sn—C0.2070(7), CC 0.1215(6), C—C 0.1460(7), C—F 0.1343(2), CCF 111.3(0.2). The uncertainties (given in parentheses) represent three times the standard deviation values. The results obtained point to practically free rotation of the CF 3 groups. The presence of electronegative CF 3 causes shortening of the Sn-C bonds in Sn(CC—CF 3 ) 4 from Me 3 SnCCH and Me 3 SnCCSnMe 3 . The triple CC bond length is larger than in hexafluoro-2-butyne and nearly the same as in dimethylacetylene.

A nonparametric determination of the internal rotation potential and the molecular geometry of o-chloroanisole using gas electron diffraction and data from spectroscopy and ab initio calculations

Abstract Structural parameters of o -chloroanisole have been determined using a dynamic model from electron diffraction together with vibrational spectroscopic data and ab initio calculations. A new approach to calculate the nonparametric torsional potential of the methoxy group based on Tikhonovs method of regularization was applied. The nonparametric torsional potential gives a two-minimum function with respect to the rotation energy around the CO bond. The most stable conformation corresponds to a planar form with the torsional angle ϕ = 0° and another to an orthogonal form with ϕ = 90°. The energy difference between these two conformers is found to be 0.9–1.0 kcal mol −1 and the barrier height at ϕ = 65° is 1.4–1.6 kcal mol −1 . These values are in good agreement with those calculated by the ab initio method. The main geometrical parameters ( r a in A, ∠ α in degrees and errors given as three times the standard deviation from least squares are as follows: r (CC) mean = 1.398(4); r (OC Ph ) = 1.358(36); r (OC Me ) = 1.426(21); r (OCl) = 1.733(4); r (CH) Ph = 1.086(6); r (CH) Me = 1.095(6); ∠CC O C Cl = 118.7(2.2); ∠C O CC = 119.9(2.5); ∠C O C Cl C = 121.5(1.1); ∠COC = 117.6(2.6); ∠C O CCl = 119.1(2.1); ∠CCO = 124.7(1.2). The structural results are compared with those obtained for similar compounds. Stereochemical peculiarities in ortho derivatives of anisole which result in the existence of an orthogonal conformer are discussed.

A comparison of amplitudes and shrinkage corrections for C6Cl3(NO2)3 calculated using conventional and new procedures

Abstract A harmonic force field for C6Cl3(NO2)3 has been estimated, and the calculated and experimental vibrational frequencies are compared. The vibrational amplitudes and shrinkage corrections have been calculated using the conventional technique based on normal coordinate analysis (1) and the new technique of taking into consideration non-linear terms in the transformation between the Cartesian and internal coordinates (2). The shrinkage corrections for bonded distances calculated using technique (2) are substantially (an order of magnitude) smaller than those calculated with the conventional procedure (1). The opposite effect is observed for non-bonded distances depending on internal rotations, which is more consistent with the physical sense of the influence of vibrational motions on the molecular structure.

Gas phase electron diffraction study of the molecular structure of 6,6-dinitro-2,2-diphenic acid

Abstract The 6,6′-dinitro-2,2′-diphenic acid molecule has been studied by gas phase electron diffraction. The structure analysis (based on C i symmetry for the molecule as a whole and D 6h , C 2v and C s symmetries for the phenyl rings, the nitro and carboxyl groups, respectively) gives the following parameters (bond lengths, r a , in A; angles in degrees): ; CC ⪕ = 1.505 (14); CO = 1.212(8); CO = 1.409(21); CCO = 126.6(1.6); CCO = 111.8(1.3); the angle between the planes of the two phenyl rings, 71.2(0.7); the torsional angle of nitro groups, 30.4(1.4); the torsional angle of carboxyl groups, 25.1(1.5). The uncertainties given in parentheses represent three times the standard deviation values. The results are compared with the structural data on biphenyl derivatives, monomers and dimers of organic acids. The observed distances between the oxygen atoms of nitro groups and the oxygen atoms of hydroxyl groups are 2.693(26) A which may offer strong support for intramolecular hydrogen bonding.