Alfred H. Lowrey

Electron Diffraction Investigation of Dimethyl Diselenide

The molecular structure of dimethyl diselenide has been determined by an electron diffraction investigation of the vapor. A computerized background correction routine, satisfying the positivity and area criteria, was employed to reduce the data and obtain a final molecular intensity curve. The values for a set of nine distances and amplitudes were obtained from a least‐squares fit to the final molecular intensity curve. The bonded distances are rg(C–H) = 1.131 ± 0.008 A, rg(C–Se) = 1.954 ± 0.005 A, and rg(Se–Se) = 2.326 ± 0.004 A. The corresponding amplitudes are l(C–H) = 0.079 ± 0.007 A, l(C–Se) = 0.054 ± 0.003 A, and l(Se–Se) = 0.056 ± 0.002 A. The ∠CSeSe = 98.9 ± 0.02° and the ∠HCSe = 108.4 ± 0.8°. The errors are estimated to be at the 99% confidence level. The methyl groups are unsymmetrically placed with respect to the CSeSe planes. They are rotated about the C–Se bonds such that one of the HCSe planes in each CH3Se moiety makes an angle of 36.1 ± 6° with the respective CSeSe plane. Dimethyl diseleni...

Quantum chemical descriptors for linear solvation energy relationships

Linear solvation energy relationships (LSER) have been successfully used to correlate over 300 complex chemical and biochemical properties with small sets of descriptors related to fundamental characteristics of molecular structure and chemistry. This is a branch of the unnamed science (after Hansch) which is concerned with the intricate interactions between chemicals and life. Until recently, the descriptors in these relationships have been empirically determined. Recent developments in both the experimental understanding of these descriptors and in molecular orbital calculations have provided a new ground for fertile interaction between computational and experimental techniques. This manuscript describes development in the use of LSER and the recent molecular orbital formulations which provide basic descriptors developed from fundamental computational techniques. Examples from current research are presented to illustrate the expanding areas of chemistry accessible to these new ideas.

Estimation of the torsional potential for perfluorodi-methyl ether from electron-diffraction data

Abstract A structural model is obtained for perfluorodimethyl ether, based on the assumption of an equilibrium geometry with the effects of internal rotation appearing in the observed vibrational amplitudes. By separation of frame vibrations obtained from a spectroscopie force field, explicit formulae for calculating the torsional contributions from single and double rotors indicate a three-fold barrier height of 6.0 ± 1.5 kcal mol −1 . With this barrier height, the observed torsional displacement away from the COC equilibrium plane is found to be a significant feature of the model.

The molecular structure of mesitylene as determined by electron diffraction

Abstract The investigation of ring deformations in benzene derivatives has revealed important structural consequences of substituent effects [1–3]. The associated merits and difficulties of the various physical techniques used to determine ring deformations have been discussed [4]. Electron diffraction has proved suitable for the investigation of symmetrically para -disubstituted benzene derivatives [5–7] and is expected to be equally applicable to sym -trisubstituted derivatives, where the (planar) ring geometry is entirely characterized by one bond length and one angle. As a continuation of our studies on benzene derivatives, including toluene and p -xylene [6], the molecular structure of mesitylene (1,3,5-trimethylbenzene) in the gas phase was determined by electron diffraction. This compound was the subject of early “visual” electron diffraction studies [8,9] in which the benzene ring was assumed to be undistorted.

Scaled quantum mechanical force field for glycine in basic solution

We obtain scale factors for three glycinate-nH2O ab initio force fields, using the 4–31G basis set, that can be used in building a scaled quantum mechanical force field for alanine and, subsequently, for peptides in aqueous solutions. Force constants from the fully optimized glycinate-nH2O supermolecules were scaled by using experimentally determined vibrational frequencies of glycine in water at pH 13. Similar calculations were performed for methylamine and acetate. Scale factors for the stretching modes of acetate are within 2% of the related scale factors for glycinate. The scale factor for the NH2 scissor mode in methylamine is also in agreement with that of glycinate. Changes in the scale factors as a function of the number of hydrating water molecules were also similar between glycinate and acetate. Amine groups showed relatively small changes. Scale factors for glycinate with no hydrating molecules were extrapolated from the supermolecule results, since the optimized structure of isolated glycinate obtained with the 4–31G basis set yielded one imaginary frequency. Good agreements between calculated and experimental frequencies for glycinate, acetate, and methyl amine were obtained for each set of scale factors. Scaling appears to compensate for the systematic effects of hydration on force constants, making it possible to obtain reliable frequency predictions for amino acids in water without resorting to expensive super-molecule calculations.

Effects of hydration on scale factors for ab initio force constantsII

Abstract Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To achieve this goal, it has been necessary to obtain scale factors for smaller hydrated molecules that can be used as a starting point for calculations on peptides. To this end, we have calculated scale factors for ab initio force constants for methylamine and protonated methylamine using a least-squares fit of calculated and experimental frequencies. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. We compare the scale factors derived from our measurements with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G, 4-31G**, 6-31G+, and 6–311G ∗∗ basis sets. The results here are similar in consistency with those found in our previous calculations on formic acid, acetic acid and acetone. The differences in scale factors between vapor and aqueous phase molecules are smaller than those previously found for polar groups, indicating that methylamine exhibits relatively non-polar behavior in solution with respect to its vibrational spectrum.

The molecular structure and torsional barrier of tri-fluoroacetyl fluoride in the gas phase

Abstract The molecular structure and rotational barrier of trifluoroacetyl fluoride in the gas phase have been determined by electron diffraction. The geometrical parameters are: r α (C-C): 152.2 ± 0.6 pm; r (C-F): 132.4 ± 0.2 pm; r (CO): 115.8 ± 0.7 pm; ∠(C-C-F): 109.5 ± 0.5° ; ∠(C-CO): 129 ± 2°. The torsional barrier is adequately described by a threefold potential, V 3 = 7 ± 2 kJ mol −1 .

Molecular structure of 1,1‐dichloro‐2,2‐difluoroethylene

The molecular structure of 1,1‐dichloro‐2,2‐difluoroethylene (CCl2=CF2) has been determined by an electron diffraction study in the vapor phase. A Euclidean structural model was obtained from the refined unconstrained least‐squares distances with a procedure that varies the coordinates of a Euclidean model until the best least‐square fit to the non‐Euclidean distances is obtained. This procedure provides a means for estimating shrinkage effects due to molecular vibrations. The bonded distances obtained are rg(C–F) =1.315±0.015 A rg(C=C) = 1.345±0.025 A, rg(C–Cl) =1.706±0.008 A; the associated vibrational amplitudes are l (C–F) =0.049±0.018 A, l (C=C) = 0.044±0.028 A, l (C–Cl) =0.055±0.009 A. The &FCF angle is 112.1°±2.5° and the &ClCCl angle is 119.0°±0.9°. The errors are limits of error at the 95% confidence level. The results indicate a negligible cis Cl⋅⋅⋅F shrinkage (Δra=0.001 A) and an appreciable trans Cl⋅⋅⋅F shrinkage (Δra=0.01 A). The unsymmetrical halogen substitution has caused the C=C distance ...

Electron diffraction investigation of 1‐methoxycyclohexene

An electron diffraction investigation of 1‐methoxycyclohexene was undertaken to determine if the molecule possesses a favored conformation for the vinyl ether portion. In the gas phase, the molecule was found to be predominantly, if not entirely, in the cis conformation. In the cyclohexene ring, the bonded parameters are found to be C=C=1.333± 0.009 A (lC=C=0.048± 0.007 A), C2–C3=1.503± 0.006 A (lC–C=0.051± 0.005 A), C3–C4=1.514± 0.006 A (lC–C=0.042± 0.005 A), C4–C5=1.539± 0.014 A (lC–C=0.041± 0.009 A), the single bonds becoming longer the larger the separation from the double bond. The bonded parameters associated with the methoxy group are C–O=1.364± 0.006 A (lC–O=0.039± 0.005 A) for the distance from the ring to the carbon atom and C–O=1.421± 0.006 A (lC–O=0.041± 0.005 A) for the distance from the oxygen atom to the methyl carbon atom. The ring angles are C6C1C2=124.2± 1.0°, C1C2C3=123.1± 1.0°, C2C3C4=112.1± 1.0°, C3C4C5=111.5± 1.0°. The angles associated with the methoxy group are OC1C2=122.5± 1.5°, C...

Using theoretical descriptors in quantitative structure-activity relationships: HPLC capacity factors for energetic materials

The widespread application of computational techniques to studies in biology and chemistry has led to a quest for important characteristic properties that may be directly derived from these methods. The theoretical linear solvation energy relationships (TLSER) have successfully replaced empirical parameters, such as in the linear solvation energy relationships (LSER) of Kamlet and Taft, with theoretical descriptors that are consistently derived for a wide variety of chemical and biological properties and for a large range of molecules. These descriptors are small in number, well-defined in theoretical terms, and provide a systematic basis for computational studies of complex phenomena. Energetic materials are a class of molecules whose explosive characteristics have been previously studied by empirical methods. The forensic and environmental sciences have prompted the conduct of detailed high performance liquid chromatography (HPLC) studies for identifying of trace quantities of commonly used explosives. The TLSER methodology provides good correlations between the calculated quantities and the experimentally determined HPLC capacity factors. This provides a reasonable interpretation of retention times in terms of molecular volume and quantities associated with acidity and basicity.