LCAO-MO Calculations on Boron Compounds

10 LCAO-MO Calculations on Boron Compounds I. Aminoboranes JOYCE J . K A U F M A N and J O N R. H A M A N N

Downloaded by EAST CAROLINA UNIV on January 13, 2018 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch010

Research

Institute

for Advanced

Studies,

7212 Bellona

Ave., Baltimore

12, Md.

The primary purpose of this study was to reproduce reported experimental observations for a series of heteroatomic boron compounds by simple L C A O - M O Hückel calculations, employing as input data only basic atomic or group parameters derived without reference to the particular molecular environments considered here. Hückel calculations for aminoboranes (using the four possible input parameter sets derivable from other molecules) were compared, point by point, with experimental observations of seven aspects of the behavior of aminoboranes upon substitution. Each experimental observation was correctly predicted. More important, the trend of the calculational results is insensitive to the choice among the input parameter sets. R igorous Pariser-Parr and Pople self-consistent field (SCF) calculations for borazines and heteroaromatic boron compounds have been reported (2,10). Calculations of this kind are tedious, expensive, and difficult, requiring elaborate computer programs and much computer time, as well as a fairly precise knowledge of the molecular geometries and bond distances of the compounds. In view of the considerable success which the much simpler Hückel calculations have had in the field of organic π-systems, it was felt desirable to attempt to reproduce reported experimental observations for a series of heteroatomic boron compounds by theoretical calculations into which no preconceived notions of the molecular bonding to be ex pected were incorporated. Many new classes of boron heteroatomic com pounds, some even previously unimagined, are being prepared today. To hope for success in developing Hückel procedures general enough to treat the planar portions of these boron-containing molecules (already discovered and as yet undiscovered), it is imperative to be able to use as input data only basic group parameters derived without reference to the molecular environment in which they later find themselves. As preliminary preparation, linear combination of atomic orbitals-molecular orbital (LCAO-MO) calculations for a wide variety of heteroatomic boron compounds were performed by the Hückel technique. 95

Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

96

ADVANCES

IN

C H E M I S T R Y SERIES

The main emphasis was placed on derivation and evaluation of input parameters, especially for substituent groups in such compounds as substitutedborazines and boroxines, substituted heteroaromatic boron compounds, and substituted aminoboranes. An indication for the choice of input parameters and the theoretical justification for such choices was derived from earlier work on δ values (6-9,11) [the effect of substituent groups on properties of free radicals and molecules of B, C, N, O, and S compounds]. The series of molecules chosen for comparison of calculated and observed molecular properties was the aminoboranes, R R B - N R " R " \ for which the molecular geometries are so imprecisely known that Pariser-Parr and Pople calculations are infeasible. κ

T


Details

of

Calculations

The Hiickel calculational procedure for π-systems is described in standard texts on quantum chemistry (4,14). The basic input elements are the diagonal Coulomb integrals, a, and the off-diagonal resonance integrals, β. a may be regarded as propor tional to the effective valence state ionization potential of the π-electron and for heteroatoms a = ŒQ + values of β for linkages of all other atom pairs are expressed as βχγ = ι?χγ ^ Ç Q , where η may be considered approximately proportional to the overlap integral, 5 χ γ . The choice of values for < 5 and η has been the subject of numerous articles. For common atomic linkages and substituent groups a collection of "standard" values has been assembled (12). For less common linkages, evaluation is often performed by fitting the results of a calculation for one such molecule to observed experimental results and then carrying the set of parameters so derived over to re lated compounds (hoping that the parameters remain fairly invariant). This was the procedure followed in the present study. α and had originally been evaluated by Mulliken (13) from con sideration of borazine: X

x

a

n

d

B

« B a

N

=

=

C " " c C

a

û

?

C

+

^ C C

0ΒΝ evaluated by Kubo (15) as 0.87 βçQ from consideration of the B N overlap integral in borazine. 0 had also been estimated as 0.57 /?cc by Dewar (5) by fitting the experimentally observed highest occupied molecular orbital of X, DC-borazarophenanthrene to the results of a calculation, using the above values for α g and ot^. These two values for 0 span approximately the range from almost neutral B - N toB~ - N+. They should presumably bracket the value for β-Q^ in any intermediate situation. Consequently, calculations were carried out utilizing each of the above values of β^ to ascertain the sensitivity of the results to a choice of β^' A l l of the aminoboranes calculated in this study contained substituent groups on both the boron and nitrogen atoms. For that reason, it was necessary to consider how the integrals for substituent groups would be affected by substitution of these groups for an H on a Β or Ν atom of a π-system, rather than for the H on a C atom of a π-system. Earlier w

a

s

B

B

N

N


70

KAUFMAN

Molecular

Orbitals of Aminoboranes

97

research on àjç values had shown that a substituent group produced an almost identical effect on the π-electron of boron, carbon, or nitrogen (6-9,11). Therefore, it should be possible to utilize the standard Hiickel parameters for the substituted methyl or phenyl groups on the Β or Ν atoms of aminoborane. There are two sets of parameters in current usage for substituent methyl groups. One includes an inductive effect of the methyl group on the atom to which it is attached (1); the other does not (3). The trend of the calculational results is insensitive to the choice among the four input parameters sets. The experimental data are therefore compared to only one set of calculational results — that using ^BN °- 87 ^CC i °* n inductive effect due to the methyl group. =


AND HAMANN

w i t h

Results

and

n

c

l

u

s

i

o

n

a

Discussion

The experimental observations chosen to be reproduced by the the oretical calculations were those reported by Wyman, Niedenzu, and Dawson (16) in an article on the infrared spectra, bond character, and hydrolytic stabilities of organo-substituted aminoboranes. The first observations to be compared with theoretical calculations concern the introduction of N-phenyl groups. There are three infer ences from experimental data which can be checked against the calcu lational results presented in Table I, where data are so arranged that the substituent groups on the boron are kept constant while a phenyl group is substituted for a hydrogen atom or a methyl group on the nitro gen. Each pointtobe compared is listed separately, followed directly by the discussion pertaining to it. Table I.

Bond

Substituents Ν

iV-Phenylaminoboranes

B-N

Β

Charges

Orders N-Ph

Β

Ν

Me, Η

Me

2

0.5975

—

0.2930

1.7394

Ph, Η

Me

2

0.5454

0.3811

0.2744

1.6755

0.2930

1.7394

0. 3072

1.6904

Me, Η

Me

2

0.5975

Me

Me,

0.6017

— —

Me

0.5604

0.2806

0.2897

1.6538

Ph, Me

0.5926

—

0.3885

1.6690

Ph, Me 2-(p-MePh)

0.5397 0.5764

0.3916

0.3711 0.4662

1.6119 1.6543

2-(£-MePh)

0.4853

0.3742

0.4428

1.5602

2

Ph, Me Me

2

Ph, Me Me 2

P h

2

2

"Introduction of an N-phenyl group brings about considerable lower ing of the double bond character of the B - N linkage" (16). Results of the calculations clearly indicate a lowering in the B - N bond order upon substitution of a phenyl group for a hydrogen atom or a methyl group on nitrogen. Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

98

ADVANCES

IN

C H E M I S T R Y SERIES

"This lowering of B - N bond order is caused by resonance interac tion of the unshared electron pair of the nitrogen atom with the adjacent phenyl group" (16). Two aspects of the calculations verify this con clusion: There is a sizable calculated N-Ph bond order, and the cal culated electron density on nitrogen is lowered by introduction of a phenyl group onto the nitrogen. "Such interaction results in increased electron deficiency at the boron atom" (16). In each case the calculated electron density on the boron atom is lower for the JV-phenyl-substituted compound. The second set of observations concerns the introduction of B - a r y l groups. The necessary calculations are presented in Table II.


Table Π. Β -Arylaminoboranes Substituents

Bond Orders Β

Ν Me

2

Me

2

Me

2

Me

2

Β

Ν 1.6579

0.6017

— —

0.3072

1.6904

Ph, Me

0.5926

0.3641

0.3885

1.6690

2-#>-MePh)

0.5764

0.3642

0.4662

1.6543

0.5604

—

0.2897

1.6538

0.5397

0.3709

0.3711

1.6119

H

2

Ph, Me

B-Ph

0.2836

2 Me

Ph, Me

B-N

Charges

Me

2

Ph, Me

0.6674

"Resonance interaction with the aryl group is again possible, lead ing to an increased double-bond character for the ^-phenyl bond" (16). The calculations indicate a sizable B-Ph bond order. "This resonance interaction leads to a corresponding decrease in double-bond character for the B - N link" (16). In each case, upon sub stitution of an aryl group on to boron the calculated B - N order is les sened. "The effect of B-aryl groups seems less pronounced than that of JV-phenyl groups" (16). The lowering of the calculated B - N bond order for jB-phenyl substitution is considerably less than that for JV-phenyl substitution. "The electron density around the boron atom is not diminished" (16). On the contrary, the calculated charge on boron shows that the electron density on the boron atom is raised by aryl substitution. This is due to the "amphoteric" character of the phenyl group, which allows it to donate electrons to an electron-deficient center or attract them from an elec tron-rich center. Conclusions Comparison of results of these calculations with reported experi mental observations proves that the Hiickel technique can be applied with considerable success to at least certain types of heteroatomic boron compounds. The trend of the calculational results is (happily'.) insensi tive to the particular choice among the four proposed sets of input parameters. The results based on the various parameter sets show merely a regular modest over-all difference in the calculated charges and bond orders. Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

70

KAUFMAN


Literature

AND

HAMANN

Molecular

Orbital

of Aminoboranes

99

Cited

(1) Berthier, G., Faculty of Science, University of Paris, Paris, France, private communication [corrects typographical error for methyl group in (12)]. (2) Chalvet, O., Daudel, R., Kaufman, J. J., Advan, Chem. Ser., No. 42, 251 (1963). (3) Coulson, C. A., Crawford, V. A., J. Chem. Soc.,1953, 2052. (4) Daudel, R., Lefebvre, R., Moser, C., "Quantum Chemistry. Methods and Applications," p. 52, Interscience, New York, 1959. (5) Dewar, M. J . S., Lepley, A . R., J. Am. Chem. Soc. 83, 4560 (1961). (6) Kaufman, J. J., Ibid., 84, 4393 (1962). (7) Ibid., 85, 1576 (1963). (8) Kaufman, J. J., J. Phys. Chem. 66, 2269 (1962). (9) Kaufman, J. J., Research Institute for Advanced Studies, RIAS Tech Rept. 62-22 (1962); J. Am. Chem. Soc. 85, 1576 (1963). (10) Kaufman, J. J., Hamann, J. R., Advan. Chem. Ser., No. 42, 95 (1963). (11) Kaufman, J. J., Koski, W. S., J. Am. Chem. Soc. 82, 3262 (1960). (12) Pullman, B., Pullman, A., Rev. Mod. Phys. 32, 428 (1960). (13) Roothaan, C. C. J., Mulliken, R. S., J. Chem. Phys. 16, 118 (1948). (14) Streitwieser, A., "Molecular Orbital Theory for Organic Chemists," p. 33, Wiley, New York, 1961. (15) Watanabe, H., Ito, K., Kubo, M., J. Am. Chem. Soc. 82, 3294 (1960). (16) Wyman, G. M., Niedenzu, K., Dawson, J . W., J. Chem. Soc. 1962, 4068. Received May 27, 1963. Research supported by the A i r Force Office of Scientific Research, Office of Aerospace Research, under Contract No. A F 49(638)-1220.


LCAO-MO Calculations on Boron Compounds

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