Douglas J. DeFrees

Self‐consistent molecular orbital methods. XXIII. A polarization‐type basis set for second‐row elements

The 6‐31G* and 6‐31G** basis sets previously introduced for first‐row atoms have been extended through the second‐row of the periodic table. Equilibrium geometries for one‐heavy‐atom hydrides calculated for the two‐basis sets and using Hartree–Fock wave functions are in good agreement both with each other and with the experimental data. HF/6‐31G* structures, obtained for two‐heavy‐atom hydrides and for a variety of hypervalent second‐row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first‐row compounds, where limiting Hartree–Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6‐31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split‐valance 3‐21G basis set for both normal‐ and hypervalent compounds. Normal‐mode vibrational frequencies derived from 6‐31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%–15%; they are of much more uniform quality than those obtained from the 3‐21G basis set. Hydrogenation energies calculated for normal‐ and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences.

Structures of C4

Abstract Linear ( 1 ), cyclic ( 2 ) and bicyclic ( 3 ) alternatives are considered as possible ground-state structures for C 4 . At the highest levels of theory, MP4SDQ/6-31 ∗ //HF/6-31 ∗ , 3, with two π electrons is found to be most stable.

The quantum mechanical calculation of rotational spectra. A comparison of methods for C2H2, HCN, HNC, HCO+, N2H+, CO and N2. Predictions for HCNH+, COH+, HBO, HBNH, and HBF+.

Rotational frequencies determined with ab initio molecular orbital theory can play an important role in guiding spectroscopic searches for new molecules and in corroborating the assignment of unidentified lines, from the laboratory and from space. In a systematic study of 22 levels of molecular orbital theory, CISD/6-311G** gave rotational frequencies to an accuracy of +/- 0.4 GHz when an empirical correction is applied to the results for C2H2,HCN, HNC, HCO+, N2H+, CO, and N2. Larger errors can be expected when there are large vibrational effects on the rotational constants, as exemplified by COH+. Predicted J = 0--> 1 rotational frequencies using these methods are 73.9 +/- 0.4 GHz for HCNH+, 78.6 +/- 0.4 GHz for HBO, 65.8 +/- 0.4 GHz for HBNH, and 72.1 +/- 0.4 GHz for HBF+.

Is N‐protonated hydrogen isocyanide, H2NC+, an observable interstellar species?

Ab initio molecular orbital theory is used to examine the singlet and triplet potential energy surfaces for the CH2N+ system. The results confirm those of earlier studies which suggested that the singlet H2NC+ isomer could be formed via the corresponding triplet isomer. Also, it is shown that the reaction HCN+ + H2 might lead to this metastable isomer without invoking the triplet species. The best test of the hypothesis that this molecule can be formed by gas phase, ion molecule reactions and may be an important precursor in the interstellar synthesis of HCN and HNC is to search for it in space. To this end, theoretical predictions are made of its rotational frequencies and its vibrational frequencies and intensities to serve as a guide to laboratory spectroscopists and radioastronomers.

A THEORETICAL STUDY OF THE FLUOROHYDROXY BORANES BFN(OH)3‐N

Abstract The stepwise hydrolysis of boron trifluoride, BF 3 , to boric acid, B(OH) 3 , is studied with ab initio molecular orbital theory. Restricted Hartree—Fock geometries computed with the 3-21G basis set for BF 3 , BF 2 OH, BF(OH) 2 , and B(OH) 3 are in satisfactory accord with experimental structures although BF and BO bondlengths are systematically over-estimated. The overall hydrolysis reaction is computed ab initio to be endothermic by 16.9 kcal mol −1 at STP, with the energy change occurring primarily at the Hartee—Fock with smaller contributions from the correlation and vibrational energies. Each of the three hydrolysis steps contributes approximately equally to the overall endothermicity.