MP2 Calculation of Approximate Equilibrium Molecular Structures.
	Accurate calculation of nuclear quadrupole coupling constants in gaseous state molecules requires, of course, accurate molecular structures on which to make the calculation.
	Calculation of near equilibrium molecular structures requires a high level of ab initio theory - for example, CCSD(T) - in conjunction with large bases - for example, cc-pVTZ or larger; which in turn requires computer resources beyond those available for this work.
	However, for calculation of approximate equilibrium structures, Demaison et al. have shown in a series of publications [1 - 8] that errors inherent in more modest quantum chemistry calculation of bond lengths are largely systematic and can be empirically corrected, and that accurate interatomic angles may be obtained at the MP2 level of theory in conjunction with sufficiently large triple-zeta bases.
	Applying the methods of Demaison et al., MP2/6-311+G(d,p) optimization was made of a number of molecules containing C-C, C=C, C≡N, CF, and/or C=O bonds, and MP2/6-311+G(2d,p) optimization of a number of molecules containing CCl and/or CBr bonds, for which equilibrium (re or rmrho) structures are known.  Linear regression analyses of the optimized bond lengths versus the equilibrium bond lengths yield regression equations that may be used for correction of the optimized bond lengths.
	Thus, the following equations have been derived.  Visit the links for details.
	C-C ~ re(Å) = 0.93958 × ropt + 0.08442 RSD = 0.0016 Å   C=C ~ re(Å) = 0.93285 × ropt + 0.08089 RSD = 0.0014 Å   C≡N ~ re(Å) = 0.59767 × ropt + 0.45394 RSD = 0.0009 Å   CF ~ re(Å) = 0.96166 × ropt + 0.04418 RSD = 0.0019 Å   CCl ~ re(Å) = 0.99534 × ropt - 0.00877 RSD = 0.0029 Å   C=O ~ re(Å) = 1.06958 × ropt - 0.09136 RSD = 0.0024 Å   CBr ~ re(Å) = 0.99938 × ropt - 0.00918 RSD = 0.0009 Å
	The standard deviation of the the residuals (RSD) may be taken as an estimate of the uncertainty in the calculated bond length, ~ re.
	For CH bond lengths [6]
	~ re(Å) = 1.001 × ropt, where ropt = MP2/6-31G(d,p)
	Calculations were made on a Mac G5 with the G03M quantum chemistry package of Gaussian Inc.
	[1] J.M.Colmont, D.Priem, P.Dréan, J.Demaison, and J.E.Boggs, J.Mol. Spectrosc. 191,158(1998).
	[2] J.Demaison, G.Wlodarczak, H.Rück, K.H.Wiedenmann, and H.D.Rudolph, J.Mol.Struct. 376,399(1996).
	[3] I.Merke, L.Poteau, G.Wlodarczak, A.Bouddou, and J.Demaison, J.Mol.Spectrosc. 177,232(1996).
	[4] R.M.Villamañan, W.D.Chen, G.Wlodarczak, J.Demaison, A.G.Lesarri, J.C.López, and J.L.Alonso, J.Mol.Spectrosc. 171,223(1995).
	[5] J.Demaison, J.Cosléou, R.Bocquet, and A.G.Lesarri, J.Mol.Spectrosc. 167,400(1994).
	[6] J.Demaison and G.Wlodarczak, Struct.Chem. 5,57(1994).
	[7] M.LeGuennec, J.Demaison, G.Wlodarczak, and C.J.Marsden, J.Mol.Spectrosc. 160,471(1993).
	[8] M.LeGuennec, G.Wlodarczak, J.Burie, and J.Demaison, J.Mol. Spectrosc. 154,305(1992).
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		Last modified: 27 Nov 2006