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Ab initio calculation; Equilibrium structure; Fundamentals; Anharmonic force fields; Spectroscopic constants
Abstract:
Highly correlated ab initio calculations employing large basis sets are reported for monomeric methyllithium, CH3Li. Combining theoretical vibrational corrections and experimental ground-state rotational constants from a recent microwave investigation of three isotopomers of methyllithium, the equilibrium geometry of CH3Li was determined: re(CLi) = 196.22(34) pm, re(CH) = 109.58(14) pm, and ∠e(HCH) = 105.69(24)°. The error bars are chosen such that a purely theoretical estimate of the re structure is also covered. Correlated harmonic [CCSD(T)/cc-pVQZ + aug(C)] and anharmonic [MP2/MT(ae)] force fields provide theoretical values for the fundamental vibrational wavenumbers in CH3Li which differ significantly from those observed in an argon matrix, except for the degenerate bending modes. Possible explanations for this discrepancy are discussed. It is concluded that reliable gas phase values are needed in order to resolve this discrepancy. Many spectroscopic constants of CH3Li are predicted.
