Molecular orbital calculations, gases

The relative acidities in the gas phase can be detennined from ab initio or molecular orbital calculations while differences in the free energies of hydration of the acids and the cations are obtained from FEP sunulations in which FIA and A are mutated into FIB and B A respectively. 

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

Marriott and Topsom have recently developed theoretical scales of substituent field and resonance parameters. The former correspond to the traditional inductive parameters but these authors are firm believers in the field model of the so-called inductive effect and use the symbol The theoretical substituent field effect scale is based on ab initio molecular orbital calculations of energies or electron populations of simple molecular systems. The results of the calculations are well correlated with Op values for a small number of substituents whose Op values on the various experimental scales (gas-phase, non-polar solvents, polar solvents) are concordant, and the regression equations are the basis for theoretical Op values of about 50 substituents. These include SOMe and S02Me at 0.37 and 0.60 respectively, which agree well with inherent best values in the literature of 0.36 and 0.58. However, it should be noted that a, for SOMe is given as 0.50 by Ehrenson and coworkers . 

The bands due to Fe(CO)4 are shown in Fig. 8. This spectrum (68) was particularly important because it showed that in the gas phase Fe(CO)4 had at least two vq—o vibrations. Although metal carbonyls have broad vC—o absorptions in the gas phase, much more overlapped than in solution or in a matrix, the presence of the two Vc—o bands of Fe(CO)4 was clear. These two bands show that in the gas phase Fe(CO)4 has a distorted non-tetrahedral structure. The frequencies of these bands were close to those of Fe(CO)4 isolated in a Ne matrix at 4 K (86). Previous matrix, isolation experiments (15) (see Section I,A) has shown that Fe(CO)4 in the matrix had a distorted C2v structure (Scheme 1) and a paramagnetic ground state. This conclusion has since been supported by both approximate (17,18) and ab initio (19) molecular orbital calculations for Fe(CO)4 with a 3B2 ground state. The observation of a distorted structure for Fe(CO)4 in the gas phase proved that the distortion of matrix-isolated Fe(CO)4 was not an artifact introduced by the solid state. 

Recently, theoretical calculations were done on the olefin polymerization. In particular, an ab-initio molecular orbital calculation was used to optimize the geometry for the ground, transition and product states of model systems, based on gas-phase reactions ... 

Klamt, A. (1993) Estimation of gas-phase hydroxyl radical rate constants of organic compounds from molecular orbital calculations. Chemosphere 26, 1273-1289. 

On the other hand, electric dipolar moments of the solute molecules can be obtained with standard methods in ab initio molecular orbital calculations, whereas the induced dipole moments in solution are determined from differences between the values obtained in solution and in the gas phase. 

Weiske, T. Koch, W. Schwarz, H. Experimental Evidence for the Existence of the Protonitronium Dication (H0N02+) in the Gas Phase and ab Initio Molecular Orbital Calculations of Its Potential Energy Surface. J. Am. Chem. Soc. 1993,115, 6312-6316. 

The new treatment had its origins partly in ab initio molecular orbital calculations of substituent effects and partly in extensive studies of gas-phase proton transfer reactions from about 1980 (Section V.A). Various aspects of this work essentially drew attention to the importance of substituent polarizability. In 1986 Taft, Topsom and their colleagues252 developed a scale of directional substituent polarizability parameters , oa, by ab initio calculations of directional electrostatic polarization potentials at the 3-21G//3-31G level for a large set of CH3X molecules. The oa values were shown to be useful in the correlation analysis of gas-phase acidities of several series of substrates252, and such work has subsequently been extended by Taft and Topsom151. 

The EPR spectra of electrolytically produced anion radicals of Q -aminoanthraquin-ones were measured in DME and DMSO. The isotropic hyperfine coupling constants were assigned by comparison with the EPS spectra of dihydroxy-substituted antraquinones and molecular-orbital calculations. Isomerically pure phenylcarbene anion (PhCH ) has been generated in the gas phase by dissociative electron ionization of phenyldiazirine. PhCH has strong base and nucleophilic character. It abstracts an S atom from and OCS, an N atom from N2O, and an H atom from... 

Stabilized with respect to the amide structure, is some 30 kcal mole less stable than the O-protonated form, therefore refers to gas phase protonation. Molecular orbital calculations by Hopkinson and Csizmadia (1973) put the same difference at only 6-2 kcal mole , again ignoring solvation. 

Klamt, A. Estimation of Gas-Phase Hydroxyl Radical Rate Constants of Organic Compounds from Molecular Orbital Calculations, Chemosphere, 26, 1273-1289 (1993). 

FIGURE 10.1 Free-energy profile for the gas phase (solid line) and aqueous solution (dashed line) Sn2 reaction between CH3CI and CL, from molecular orbital calculations.15... 

The gas phase basicities at both the a- and (3-positions in five-membered heterocycles have been studied by ion cyclotron resonance equilibrium and bracketing experiments on deuteriated substrates. a-Protonation is preferred by 2.8-4.6 kcal mol-1 for both furan and thiophene as compared to 0-2.9 kcal mol 1 for pyrrole (81NJC505) and heteroatom protonation is much less favored than a-protonation. Semiempirical (MNDO) molecular orbital calculations have provided quantitative confirmation of the above conclusions. 

S n2 displacements of Cl ion from MeCl, EtCl, and CICH2CH2CI by AcO- and by HO have been studied in ab initio molecular orbital calculations at the HF/6-31+G(d), MP2/6-31+G(d), and MP4/6-31+G(d) levels.92 The gas-phase reactions of HO have no overall barrier, but there is a small overall barrier for the reactions with AcO. A self-consistent reaction-field solvation model was used to examine the 5n2 reactions between MeCl and HO and between CICH2CH2CI and AcO- in solution. A... 

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