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() = 0.93958 ropt + 0.08442 RSD = 0.0016


C=C ~ re() = 0.93285 ropt + 0.08089 RSD = 0.0014


C≡N ~ re() = 0.59767 ropt + 0.45394 RSD = 0.0009


CF ~ re() = 0.96166 ropt + 0.04418 RSD = 0.0019


CCl ~ re() = 0.99534 ropt - 0.00877 RSD = 0.0029


C=O ~ re() = 1.06958 ropt - 0.09136 RSD = 0.0024


~ re() = 0.99938 ropt - 0.00918 RSD = 0.0009


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() = 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.Dran, J.Demaison, and J.E.Boggs, J.Mol. Spectrosc. 191,158(1998).

[2] J.Demaison, G.Wlodarczak, H.Rck, 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.Villamaan, W.D.Chen, G.Wlodarczak, J.Demaison, A.G.Lesarri, J.C.Lpez, and J.L.Alonso, J.Mol.Spectrosc. 171,223(1995).

[5] J.Demaison, J.Coslou, 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).




Last modified: 27 Nov 2006