INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, VOL. XIX, 501-503 (19x1))

Ab Initio Molecular Fragment Calculations with Pseudopotentials. Some Fragments Containing

Nitrogen and Oxygen

R. GASPAR, JR.

R. GASPAR* Department of Biophysics, Medical University of Debrecen, H-4012 Debrecen, Hungary

Lehrstuhl fur Theoretische Chemie, Institut fur Physikalische Chemie, Universitat Gottingen, Gottingen, West Germany

Abstracts

Parameters for the OH ( sp2 ) and NH, (planar, sp2) pseudopotential-FSGO molecular fragments have been established in this paper. Calculations for the molecules of formic acid, formaldehyde, and formamide show good agreement with experiment.

On prtsente des parambtres pour les fragments moltculaires OH ( s p 2 ) et NH3 (plan, sp3) disignts pour des calculs de type pseudopotentiekFSG0. Des calculs pour I’acide formique, l’aldthyde formique, et I’amide formique sont en bon accord avec l’expkrience.

Wir prasentieren Parameter fur die Molekulfragmente OH (sp’) und NH3 (eben, sp’), die in Pseudopotential-FsGO-Rechnungen verwendet werden konnen. Berechnungen fur Ameisensaure, Formaldehyd, und Formamid stimmen mit den Experimentalwerten gut uberein.

Ab inirio pseudopotential molecular fragment calculations of various hydro- carbons and heteroatom containing molecules have been reported lately [ 11. Pseudopotential molecular fragments CH4 (tetrahedral), CH3 (sp’), NH3 (tetra- hedral), NH; (tetrahedral), and H 2 0 (bent) have been used so far as building blocks of larger molecules.

The primary purpose of this paper is to announce the new OH (sp’) and NH3 (planar, sp2> pseudopotential molecular fragments. These pseudopotential molecular fragments together with the previously described ones allow one to perform pseudopotential molecular fragment calculations on different carbonyl and amide compounds.

Optimized parameters of the OH (sp’) and NH3 (planar, sp’) pseudopotential molecular fragments are displayed in Table I. The parameter values listed are the results of pseudopotential FSGO calculations with pseudopotential parameters of double-zeta quality. Details of the pseudopotential FSGO procedure and the double-zeta pseudopotential parameters applied have been described earlier [2].t

In order to prove the usefulness of the newly introduced pseudo- potential molecular fragments pseudopotential molecular fragment

* Permanent address: Institute of Theoretical Physics, Kossuth Lajos University, H-4010

t The FSGO method is explained in detail in Ref. 2c. Debrecen, Hungary.

@ 1981 John Wiley & Sons, Inc. CCC 0020-7608/81/040501-03$01 .OO

502 GASPAR, JR. AND GASPAR

TABLE I. Optimized molecular fragment parameters” with the FSGO’S ( 2 / ~ p f ) ~ ’ ~ exp(-r;I/pf).

Distance from Fragment Orbital radius the heavy atom Molecular parameter

OH sp2 OH 1.1407055665 0.5289374051 ROH = 1.200 p ~ p 1.2033937433 0.009876875 Eva, = - 14.46067222 p , 1.1765237679 * O . l

N H ~ planar, sp2 p N H 1.4870732885 0.8925336819 R N H = 1.93131910 pr 1.6073806698 *0.1 Eva,= -10,17997364

a All distances and energies are given in a.u

calculations have been performed on the formic acid, formaldehyde, and forma- mide molecules.

The formic acid molecule has been assembled from the CH3 (planar), H 2 0 (bent), and OH (sp’) pseudopotential molecular fragments and the C-H bond in the molecule was represented by a CH-type Gaussian orbital of the CH, (tetrahedral) pseudopotential fragment. The formaldehyde molecule has been constructed from the CH3 (planar) and O H (sp’) pseudopotential molecular fragments with similar substitution of the C-H bond representingorbital as in the formic acid molecule. The formamide molecule has been constructed from pseudopotential molecular fragments in a similar way to the formic acid molecule. The only difference is that instead of the H20 (bent) fragment an NH3 (planar) pseudopotential fragment has been attached to the central CH3 (planar) molecu- lar fragment.

TABLE 11. Geometry and energy predictions for molecules containing C = O bond.”

Calculated value Parameter Experimental’ by pseudopotential Deviation‘

Molecule predicted value fragment method (O/O 1

Total valence energy -39.0289 -34.6840 11.13

2.3527 2.3041 2.07 C=O distance

Formic acid

Total valence Formaldehyde energy -22.9797 -20.6493 10.14

C=O distance 2.2866 2.3432 2.48

-29.8568 10.95 2.3622 2.3228 1.67

Total valence

C=O distance

-33.5281 Formamide energy

a All parameters are given in a.u. For experimental values of parameters see Ref. 3. Estimated value of total valence energies using experimental ionization potentials and

Measured from the experimental value of the parameter. bond energies.

MOLECULAR FRAGMENT CALCULATIONS 503

The results of the pseudopotential molecular fragment calculations on the above molecules are collected in Table 11. Experimental values of the parameters are also given for comparison. The total valence energy of the molecules was determined at the equilibrium geometry of the molecules by varying the relative positions of the fragments inside the investigated molecules. The calculated equilibrium value of the C=O double bond length in the molecules is also given in Table 11.

The displayed percentage deviation values of the calculated and experimental energies and bond lengths are characteristic of the pseudopotential, molecular fragment method and prove the applicability of the newly introduced OH ( s p 2 ) and NH3 (planar, sp2) molecular fragments in these type of calculations.

Acknowledgements

One of the authors (R.G.) expresses his gratitude to the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg for a grant, which enabled the author to spend one year at the Lehrstuhl fur Theoretische Chemie, Institut fur Physikalische Chemie, Universitat Gottingen as guest professor. The warm hospitality of the fellows there and especially that of Professor W. Bingel is gratefully acknowledged.

Bibliography

[ l ] R. Gaspar, Jr. and R. Gaspar, Int. J. Quantum Chem. 15, 567 (1979); 16, 57 (1979). [2] (a) R. Gisptir and R. Gkpar, Jr., Int. J. Quantum Chem. 15, 559 (1979);

(b) Acta Phys. Acad. Sci. Hung. 45, 27 (1978); (c) R. E. Christoffersen, Adv. Quantum Chem. 6, 333 (1972).

(Chemical Society, London, 1958). [3] Tables of Interatomic Distances and Configuration in Molecules and Ions, L. E. Sutton, Ed.

Received January 23, 1980 Accepted for publication April 25, 1980