Ab initio calculations of geometries

A. Schmiedekamp, D. W. J. Cruickshank, S. Skaarup, P. Pulay, I. Hargittai, J. E. Boggs, Investigation of the Basis of the Valence Shell Electron Pair Repulsion Model by ab Initio Calculation of Geometry Variations in a Series of Tetrahedral and Related Molecules. J. Am. Chem. Soc. 1979, 101, 2002-2010. 

Ab initio calculations of geometry of radical with SiHj instead of Si(CH3)3 group. 

Schmiedekamp A, Cruickshank DWJ, Skaamp S, Pulay P, Haigittai I, Boggs JE (1979) Investigation of the basis of the valence shell electron pair repulsion model by ab initio calculation of geometry variations in a series of tetrahedral and related molecules. J Am Chem Soc 101 2002-2010... 

Acetic anhydride in the gas phase, studied by electron diffiaction and infrared spectroscopy, supplemented with ab initio calculations of geometries and force fields ... 

Pulay P 1969 Ab initio calculation of force constants and equilibrium geometries in polyatomic molecules. I. Theory/Wo/. Phys. 17 197... 

Now, we discuss briefly the situation when one or both of the adiabatic electronic states has/have nonlinear equilibrium geometry. In Figures 6 and 7 we show two characteristic examples, the state of BH2 and NH2, respectively. The BH2 potential curves are the result of ab initio calculations of the present authors [33,34], and those for NH2 are taken from [25]. 

Fig. 5.35 Geometry predicted by CASSCF ab initio calculations of the two possible transition structure geometries for the Diels-Alder reaction between ethene and butadiene. (Figure adapted from Houk KN, J Gonzalez and Y Li 1995. Pericyclic Reaction Transition States Passions and Punctilios 1935-1995. Accounts of Chemical Research 28 81-90.)...

Ab Initio Calculation of Force Constants and Equilibrium Geometries in Polyatomic Molecules. I. Theory P. Pulay... 

An informative IR spectrum of the t-butyl radical, containing 18 bands, has been recorded after freezing of the products of vacuum pyrolysis of azoisobutane [110] and 2-nitrosoisobutane [111] in an argon matrix at 10 K (Pacansky and Chang, 1981). This spectrum is in agreement with a pyramidal structure of the radical (CH3)3C (symmetry C3v) which has elongated CH bonds in positions trans to the radical centre. On the basis of experimental vibrational frequencies and ab initio calculations of the radical geometry the enthalpy value [// (300)] of its formation has been calculated as 44 kJ moP. ... 

Table 1.4 The ab initio calculation of the AE of the CO molecule (kJ/mol) at the basis-set limit. All calculations have been carried out at the all-electron CCSD(T)/cc-pCVQZ geometry [25].

Ab initio calculations of the transition structures for the Schenck reaction of singlet oxygen suggest that Me substitution makes the transition structure earlier and more stable and that the transition-state geometry is sensitive to the position of Me substituents. ... 

Recently, the assignment of the band at 980 cm to 28 has been doubted based on new calculations (this band is shifted to 976 cm if 28 is generated from 1,4-diiodobenzene (37), which is not unusual in the presence of iodine atoms. This shift may also be attributable to the change of the matrix host from argon to neon). ° On the other hand, ab initio calculations of the IR spectrum of 28 are complicated by the existence of orbital instabilities, the effect of which may (often) be negligible for first order properties (such as geometry and energy), but can result in severe deviations for second-order properties (vibrational frequencies, IR intensities). 

The apparatus has been already described in Ref. [7]. The clusters are produced by laser vaporization of a sodium rod, with helium at about 5 bars as a carrier gas and a small amount of SF6. The repetition rate is 10 Hz. In this configuration, the vibrationnal temperature of the formed clusters is roughly 400 K,[10] that gives 85% of C2V geometry and 15% of C3V for a Boltzman distribution. The laser beams are focused onto the cluster beam between the first two plates of an axial Wiley Mac-Laren Time-Of-Flight mass spectrometer with a reflectron. The photoionization efficiency curve as well as the photoabsorption spectrum determined by a photodepletion experiement are displayed on Fig. 1(b) and 1(c) respectively. The ionization threshold is at 4.3 eV, close to the 4.4 eV calculated for the C3V isomer and 4.9 eV for the C2V isomer (see the Fig. 1 (b)). The conclusion arising out of the photodepletion spectrum shown on Fig 1(c) and from ab initio calculations of the excited states, [5] is that the observed... 

A number of MO calculations have been performed on carbonyl complexes, with methods ranging from ab initio to DVM-HFS. In any case it was found that both a donation and jr back-donation interactions are important in determining the geometrical structure and physical properties of these complexes. The ab initio calculations of Sakaki et al.110 have shown that the strengthening of jt back-donation is the driving force which stabilizes the pseudotetrahedral geometry vs. the square planar one in [Ni(PR3)2(CO)2] complexes. 

Ab Initio Calculations of 31P NMR Chemical Shielding Anisotropy Tensors in Phosphates The Effect of Geometry on Shielding... 

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