J. Chem. Inf. Comput. Sci. 1998, 38, 313-316
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Analysis of Permanent Electric Dipole Moments of Aliphatic Hydrocarbon Molecules. 2. DFT Results Gyula Tasi*,†,‡ and Fujio Mizukami† Department of Surface Chemistry, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan, and Applied Chemistry Department, Jo´zsef Attila University, Rerrich B. te´r 1, H-6720 Szeged, Hungary Received September 5, 1997
The B3LYP hybrid DFT method was applied to calculate the permanent electric dipole moments of aliphatic hydrocarbon molecules. For most of the molecules, it was found that the B3LYP results are in good agreement with the calculated data published previously and with the experimental values. For molecules that contain conjugate CdC and CtC bonds, the DFT calculated values are closer to the experimental ones. The situation is the reverse for molecules containing conjugate CdC bonds. The experimental predictions of the dipole moment for 1-buten-3-yne range from 0.22 to 0.44 D, whereas DFT supports a value of ∼0.39 D. INTRODUCTION
The permanent electric dipole moment is an important physical vector quantity. By definition, the dipole moment vector points from the center of the positive charge distribution to the center of the negative charge distribution. It provides valuable information on the structure and polarity of the molecule under consideration. In the parametrization of the most widely used semiempirical quantum chemical methods, experimental dipole moments have been taken into account. The necessary condition of the goodness of a classical molecular point charge system is the reproducibility of the experimental or calculated quantum mechanical dipole moment.1 Microwave spectroscopy is the best tool for determining gas-phase experimental values.2 Generally, the components of the dipole moment vector are also available. In several cases, in addition to data referring to the ground vibrational state, information can be found for the excited states too. Recently, we studied the permanent electric dipole moments of aliphatic hydrocarbon molecules.3 We found that the RHF/6-311G**//MP2(FC)/6-311G** ab initio quantum chemistry model (Model I) affords reliable results. FC (frozen core) in parentheses means that the inner shell electrons were excluded from the electron correlation (postHF) calculations. For the 1-buten-3-yne molecule, several “experimental dipole moments” can be found in the literature: 0.22,4,5 0.46 and 0.44 D.7 According to Model I, the first value is too small.3 Particularly for aliphatic alkanes, the RHF/6-311++G** wave function at the MP2(FC)/6-311G** geometry is even better for calculating the electric dipole moment vector than the RHF/6-311G** one.3 For propane, isobutane, and gauche-butane, the calculated scalar magnitudes are 0.0853, 0.1269, and 0.0907 D, respectively. These values are in good * Author to whom correspondence should be addressed. † Department of Surface Chemistry. ‡ Applied Chemistry Department.
accordance with the experimental results of 0.084,8 0.132,9 and 0.090 D,10 respectively. Using the calculation results obtained on aliphatic alkane molecules as reference data, we performed the parametrization of an effective one-electron method.11 The present paper reports the results obtained on aliphatic hydrocarbons with the B3LYP12,13 hybrid DFT method,14 which has been proven in several areas13,15-18 to be one of the best DFT methods. For comparison, we also report the Model I3 and AM119 results. EXPERIMENTAL DIPOLE MOMENTS
Most of the experimentally determined gas-phase dipole moments referring to the ground vibrational state of the ground electronic state of molecules were taken from a reference book.5 In most cases, the values were verified from the original publications. The first column in Table 1 gives the experimental scalar values, µ(exp), together with the literature reference. In the reference book,5 the following notation is used to designate the quality of the experimental values: for A, B, C, and D, the absolute values of the estimated experimental uncertainties are
