Accurate Equilibrium Structure and Electric Dipole Moment of HC9N: Predictions on the Basis of Large-Scale Coupled Cluster Calculations

✍ Scribed by Peter Botschwina; Matthias Horn

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 72 KB
• Volume: 185
• Category: Article
• ISSN: 0022-2852
• DOI: 10.1006/jmsp.1997.7399

✦ Synopsis

NOTE

Accurate Equilibrium Structure and Electric Dipole Moment of HC 9 N: Predictions on the Basis of Large-Scale Coupled Cluster Calculations DEDICATED TO PROFESSOR HANS BU ¨RGER ON THE OCCASION OF HIS 60TH BIRTHDAY Cyanooctatetrayne ( HC 9 N ) , a long linear molecule, was first ob-We will also quote results obtained by second-order Rayleigh -Schro ¨dinger perturbation theory with energy decomposition according to served by radio astronomy in the interstellar medium almost 20 years ago ( 1 ) . Its detection in the molecular ridge of Heile's Cloud 2 was Møller and Plesset ( 13 ) as well as the underlying SCF results. All valence electrons were correlated in the present post-Hartree -Fock cal-based on a calculated rotational constant ( 2 ) rather than a laboratory spectroscopic study. It took more than 10 years until its rotational spec-culations. Throughout, the MOLPRO96 suite of programs ( 14, 15 ) has been employed. trum could be recorded in the laboratory by means of a Fourier-transform microwave spectrometer ( 3 ) .

For the geometry optimizations we employ a basis set of 239 contracted Gaussian-type orbitals ( cGTOs ) which corresponds to Dun-Ab initio calculations for HC 9 N are scarce. We are only aware of the results of Hartree -Fock ( HF ) self consistent field ( SCF ) calculations ning's correlation-consistent polarized valence triple ( cc-pVTZ ) basis ( 16) exclusive of f functions at the carbon and nitrogen nuclei and of with relatively small basis sets ( 4 -7 ) . SCF calculations with the 4-31G and 6-31G ( d ) basis sets yielded equilibrium dipole moments of 05.46 d functions at the hydrogen nucleus. The same basis set has been used

in our previous study of HC 7 N, HC 7 NH / , and the two lowest electronic ( 6 ) and 05.52 D ( 7) , respectively, with the negative end of the dipole at the nitrogen site. The ground-state value was estimated to be 04.84 states of the C 7 N radical ( 9) . Under the reasonable assumption of linear equilibrium structures for all methods employed we obtained the equi-D (6) and 05.00 D ( 7) . These values were obtained from least-squares fits of the theoretical m e and experimental m o values of cyanopolyynes, librium bond lengths given in Table 1. According to our previous experience ( see, in particular, Ref. 9 ) , the errors in the CCSD-T and CCSD ( T ) with the latter available up to HC 5 N.

In this note we will report on the results of large-scale coupled cluster calculations are quite systematic. We will apply the same corrections to the CCSD ( T ) equilibrium bond lengths as for HC 7 N (9) . The resulting calculations with the aim to predict an accurate equilibrium structure for HC 9 N ( bond lengths accurate to 0.001 A ˚or better ) and to provide corrected equilibrium geometry is given in the last line of Table 1.

Therefrom, an equilibrium rotational constant of B e Å 289.9 MHz is an estimate of the ground-state electric dipole moment with an uncertainty of ca. 0.02 D. This paper may be considered a continuation of calculated which is 0.2% below the experimental ground-state value ( 1,3 ) . According to our previous work on HC 5 N and HC 7 N, a positive our recent work on HC 5 N (8) and HC 7 N (9) . We make use of coupled cluster theory with single and double excitation operators ( CCSD ( 10) ) value of this size for the difference B o 0 B e is quite reasonable.

The electric dipole moment of HC 9 N as calculated by CCSD ( T ) at plus a perturbative treatment of the effects of so-called connected triple substitutions by means of variants CCSD ( T ) ( 11 ) and CCSD-T ( 12 ) .

the recommended equilibrium structure from this work is m e Å 05.150 D. TABLE 1 Calculated Equilibrium Bond Lengths (in A ˚), Total Energies, and Equilibrium Rotational Constants for HC 9 N 191