Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

Abstract

The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r g parameters and calculated angles to r s-structures For CC single bond distances, deviations between calculated and observed parameters ( r g) are in the ranges of −0.006(2) to −0.010(2) Å for normal or unstrained hydrocarbons; −0.011(3) to −0.016(3) Å for cyclobutane type compounds; and +0.001(5) to +0.004(4) Å for CH 3 conjugated with CO. For CO single bonds the ranges are −0.006(9) to +0.002(3) Å for CO conjugated with CO; and −0.019(3) to −0.027(9) Å for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r s — r e = −0.049(5) Å and in CH 3OCH 3 and C 2H 5OCH 3 the r s — r e differences are −0.029(5), −0.040(10) and −0.025(10) Å. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH 3 corrections are from +0.005(6) to −0.006(6) and from −0.009(2) to −0.014(6) Å; and the range for NCO is +0.012(3) to +0.028(4) Å. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) Å. For conjugated CC double bonds the correction is less positive (+0.014(1) Å for benzene). For CO double bonds the corrections are −0.004(3) to +0.003(3) Å. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of −2.3(5)° to −6.2(4)°.