B3LYP level, theoretical calculation

X = CO2R or CN). Theoretical calculation at B3LYP/6-31G //HF/STO-3G level showed that the Si-H bond dissociation energies of H-Si(l 11) and (MesSifsSi-H are very similar, which further justifies the use of the well-established radical-based reactivity of (MesSifsSiH as a model for surface reactions. 

Theoretical calculations on the cycloaddition reactions of a range of 1,3-dipoles to ethene in the gas phase have been carried out (85) with optimization of the structures of these precursor complexes and the transition states for the reactions at the B3LYP/6-31G level. Calculated vibration frequencies for the orientation complexes revealed that they are true minima on the potential energy surface. The dipole-alkene bond lengths in the complexes were found to be about twice that in the final products and binding was relatively weak with energies <2 kcal mol . Calculations on the cycloaddition reactions of nitrilium and diazonium betaines to ethene indicate that the former have smaller activation energies and are more exothermic. 

The predictive capabilities of results of theoretical calculations of isotope effects have again been questioned,94 following an experimental and theoretical study of the decarboxylation of 3-carboxybenzisoxazole at room temperature (Kemp s reaction). The experimentally determined 15N isotope effect in acetone is 1.0312 0.0006 and the 13C isotope effect (1.0448, 1.0445, 1.0472, and 1.0418 in 1,4-dioxane, acetonitrile, DMF, and water, respectively) is independent of solvent polarity even though the reaction rate is markedly solvent dependent. Theoretical models at die semiempirical (AMI, PM3, SAMI) and ab initio (up to B3LYP/6-31+ + G ) levels were all unable to predict die experimental results quantitatively. 

Ab initio theoretical calculations for the 4-methylpent-2-yl cation by Farca iu et al.115 have shown both a distorted 1 -protonated 1,1,2-trimethylcyclopropane and the open cation to be energy minima along the reaction coordinate at the B3LYP/ 6-31G level. In contrast, only the protonated cyclopropane was found to be an energy minimum in the MP2/6-31G optimization. Whereas the open cation was a transition structure at this level, a coupled cluster geometry optimization (CCSG/6-31G ) showed that the open ion is also a true energy minimum. 

The gas-phase basicity (GB) of 3-thio-5-oxo 1, 5-thio-3-oxo 2, and 3,5-dithio 4 derivatives of 2,7-dimethyl-[l,2,4]-triazepine (Figure 1) has been measured by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry and complemented with theoretical calculations. The experimental FTICR results are discussed in Section 13.14.4.1.l(i). The structures and vibrational frequencies of all stable protonated tautomers and all transition states connecting them have been obtained by means of the B3LPY density functional method, together with 6-31G basis set expansion. The final energies were obtained at the B3LYP/6-311 + G(3df,-2p) level (2002JPC7383). 

Chemical shifts of 13C CPMAS NMR spectra of 4-(4-fluorophenyl)-2- 4-[4-(3-trifluoromethylphenyl)piperazino]butyl -2,3-dihydro-lH-pyrido [l,2-c]pyrimidinel,3-dione and its hydrochloride salt were unambiguously assigned by theoretical calculations at GIAO/DFT (B3LYP/6-311 +G ) level (08JST325). Absolute energies, bond angles, and bond distances of 9,10-dimethoxy-l, 6,7,11 fc-tetrahydro-2H,4H-[l, 3]oxazino[4,3-fl]... 

Disilyne 76 is emerald green in the solid state and shows two allowed n- n absorption bands in hexane at 259 (e 10300) and 328 (e 5800) nm and two forbidden n- n absorption bands at 483 (e 120) and 690 (e 14) nm.41 Theoretical calculations for 76 at the HF/6-311G(d)//B3LYP/6-31G(d) level have shown that HOMO and HOMO-1 are two split n orbitals and LUMO and LUMO+ 1 are two split lowest 7i orbitals. The out-of-plane n orbitals HOMO and LUMO+ 1 are represented by pure (p -p )7i orbitals, whereas the in-plane n orbitals HOMO-1 and LUMO are represented mainly by (py-py)n orbitals with a weak contribution of the a (Si-Si) orbital of the central bond. The presence of the nondegenerate n and ti molecular orbitals in 76 is responsible for the UV-vis absorption band feature of 76. 

In the case of the dimethylbromonium cation 46, ab initio calculations have found two stable minima for the protosolvated superelectrophile, the bromine and carbon protonated forms, 51 and 52, respectively (Figure 3).44a Dication 51 is estimated to be more stable than 52 by about 21 kcal/mol at the B3LYP/6-31G //B3LYP/6-31G level. When dimethylbromonium cation (46) is reacted in DF/SbFs in SO2 at —78°C, no deuterium incorporation is observed into the methyl groups. This observation is in accord with the theoretical calculations in that Br-protonation is expected to be the preferred process. 

Consider for instance, 1 //-pyrazole (87). It is known that proton transfer between both nitrogen atoms (leading to tautomerism) occurs in the gas phase, in solution and, in some cases, in the solid state (depending on the substituents on the carbon atoms). Is the process an intramolecular 1,2-shift Theoretical calculations show that this is a very high-energy process, almost forbidden [91] (214 kJ mol 1 at the B3LYP/6-31G level). Then, the logical conclusion is that proton transfer in azoles involves solvent molecules (solution) or other azole molecules (solid state). 

Leszczynski et al. have shown interest in comparing experimental (matrix isolation) and calculated (B3LYP) IR data (frequency and intensity) to discuss the tautomerism of benzo-annelated pyridonone, pyrazinone, and pyrimidinone (144-148). These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds [144], The interest of Leszczynski in problems related to tautomerism, aromaticity, and proton transfer is also apparent in a study of (lH-aza-hetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs [145],... 

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