The molecular force field and polar tensor of methyl isocyanide have been determined from its gas phase vibrational frequencies and infrared intensities. Quantum chemical results from MP2(FC), B3LYP and quadratic configuration interaction calculation including single and double substitutions procedures using a 6-311 + +G(3d,3p) basis set have been used to determine the signs of the dipole moment derivatives with respect to the normal coordinates as well as estimate individual fundamental intensities of the overlapped upsilon(1)-upsilon(5) and upsilon(3)-upsilon(6) band systems. Principal component graphical representations of the A, and E symmetry polar tensor elements were useful in determining preferred sets of tensor elements. The mean dipole moment derivative (GAPT charge) of the methyl carbon in CH3NC, 0.347 e, is between the corresponding values in CH3CN, 0.110 e, and CH3F, 0.541 e. The mean dipole moment derivatives obtained here indicate the correct Is methyl carbon ionization energy as 293.35 eV which is 0.98 eV higher than the corresponding ionization energy of the terminal atom. (C) 2002 Elsevier Science B.V. All rights reserved.5913745