Isobutane, dipole moment

Isobutane, dipole moment of, 225 Isomorphic groups, 392 Isotopes, table of properties of, 470 Isotopic substitution in ESR, 379 in IR spectroscopy, 244 in microwave spectroscopy, 221-223 in NMR, 354-355... 

Because of the small difference between the electronegativities of carbon and hydrogen, alkanes have very small dipole moments, so small that they are difficult to measure. For example, the dipole moment of isobutane is 0.132 and that of propane is 0.085 Of course, methane and ethane, because of their symmetry, have no dipole moments. Few organic molecules have dipole moments greater than 7 D. 

Another estimate seemed to support an inductive contribution to deuterium IEs on the acidity of carboxylic acids.37 This IE on acidity of some carboxylic acids was attributed to an inductive effect arising from the electrostatic interaction of the C-H or C-D dipole with the negative charge of the carboxylate, as expressed in Equation (31). The derivative dpK/dfi was estimated from the effect of a C-Cl dipole on acidity, using the difference in pATas of trichloroacetic acid (0.63) and acetic acid (4.75) and the difference between the dipole moments of t-butyl chloride (2.13 D) and isobutane (-0.13 D). Next Afj, was estimated as 0.0086 D, the difference between the dipole moments of (CH3)3CD and (CH3)3CH. Thus ApK was estimated as 0.005 per D, in excellent agreement with the observed 0.014 for acetic-d3 acid. Moreover, the IE of 0.002 per D in pivalic-J9 acid is consistent with a 2.8-fold falloff factor for inductive effects. Yet those estimates depend crucially on the difference between the dipole moments of isobutane and isobutane-d, which is unusually large, amounting to 6.5% of either s total dipole moment. 

The gas-phase dipole moment of cis-l,3,5-trimethylcyclohexane, which, in the chair conformation with methyl groups equatorial, possesses symmetry, has been determined. The value of 0.25 0.01 D is close to the value of 0.27 D estimated from group-moment assignments based on propane and isobutane. 

To return to the present case, force fields have been devised that include a charge distribution in alkanes at the atomic level, and other force fields that are based on the assumption that the amount of charge distribution at the atomic level in alkanes is negligible for most purposes, and therefore electrostatics can be neglected in molecular mechanics calculations on alkanes. In our earliest molecular mechanics calculations we concluded that since charges in alkanes were certainly small, we would omit them until there was evidence that they needed to be included. It is true that alkanes do have very small dipole moments, for example, propane (0.083 and isobutane (0.132D). If one wishes to reproduce these in molecular mechanics, then one would have to include electrostatics. But to our knowledge, this is the only improvonent that electrostatics would make in the molecular mechanics of alkanes. The foreseeable improvement would be quite small, so we continue to omit electrostatics for alkanes. 

PEF400 was first optimised in unchanged form b, K, , r and e were allowed to change K, Kg and 9 were kept. Next, PEF400opt was chang-ed to PEP401 as shown in Table 9-8, and this parameter set was optimised in the same way. For these optimisations, dipole moments of propane and isobutane were Included. 

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