








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,
ccpVTZ 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 triplezeta bases. 






Applying the methods of Demaison et
al., MP2/6311+G(d,p) optimization was made of a number of
molecules containing CC, C=C, C≡N,
CF, and/or C=O bonds, and MP2/6311+G(2d,p) optimization of a number of
molecules containing CCl and/or CBr bonds, for which equilibrium (r_{e} or
r_{m}^{rho})
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. 







CC 
~ r_{e}(Å) = 0.93958
× r_{opt} + 0.08442 
RSD = 0.0016 Å 





C=C 
~ r_{e}(Å) = 0.93285 × r_{opt}
+ 0.08089 
RSD = 0.0014 Å 





C≡N 
~ r_{e}(Å) = 0.59767 × r_{opt}
+ 0.45394 
RSD = 0.0009 Å 





CF 
~ r_{e}(Å) = 0.96166 × r_{opt}
+ 0.04418 
RSD = 0.0019 Å 





CCl 
~ r_{e}(Å) = 0.99534 × r_{opt}
 0.00877 
RSD = 0.0029 Å 





C=O 
~ r_{e}(Å) = 1.06958 × r_{opt}
 0.09136 
RSD = 0.0024 Å 





CBr

~ r_{e}(Å) = 0.99938 × r_{opt}
 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, ~ r_{e}. 






For CH bond lengths [6] 






~ r_{e}(Å) = 1.001
× r_{opt}, where r_{opt }= MP2/631G(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). 











rc_mp2_pop.html 



Last modified: 27 Nov 2006 




