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The attractive rotational barrier in nitrosomethane
Volume 5, number 2
THE
ATTRACTIVE
CHEMICALPHYSICSLETTE_RS
ROTATIONAL
BARRIER
IN
I March 19?0
NLTROSOMETHANE
*
PETER A. KOLLMAN** and LELAND CoALLEN Depurfnlenf of Chemistry, Princeton University, Princeton, New Jersey 08540, U&l Received 9 December 1969 An ab initio LCAQ MO SCF calculation has been carried out on staggered and eclipsed forms of nitrosomethane. The rotational barrier found (1.05 kc&mole) is in excellent agreement with the experimental value (1.10 k&/mole). An energy component analysis shows nltrosomethane to be an attractive dominant barrier, thereby unequivocallydemonstratingthe possibility of a small scale, attractive covalent force as the controlling effect in some three fold barriers. 1. INTRODUCTION
Rotational barriers in molecuIes have been of interest to theoreticians for many years [l]. More recently, the ability to carry out high quality molecular orbital calculations has allowed one to gain a more detailed picture of the electronic changes in the molecule during internal rotation. The concepts of "attractive dominance” and %epulSive dominance” have allowed one to relate the detailed energy components of the molecular orbital calclllation to the modellike chemical concepts of attraction and repulsion [2]. Examples of three-fold barriers possessing repulsive dominance are ethane, methyl a&ne and methanol, while attractive dominance is found in acetaidehyde [3] and nitrosomethane. In both acetaldehyde (CH3-CHO) and nitrosomethane (CH3 -N-O), the oxygen is eclupsed to a methyl hydrogen. In acetaldehyde, the carbonyl hydrogen is eclipsed with respect to the methyl rotor; thus, one cannot determine whether the barrier is dominated by the eclipsing of the oxygen to a methyl hydrogen or by the staggering of the carbonyl hydrogen with respect to the methyl hydrogens. In nitrosomethane, the equilibrium geometry is determined by the attraction (or repulsion) between the nitroso oxygen and the methyl hydrogen&
2. COMPUTATIONAL METRY
METHOD
AND GEO-
A finite expansion molecular orbital calculation (LCAO MO SCF) was carried out using essentially Hartree-Fock quality atomic orbitals as a basis set. The s orbitals were taken from Whitten’s [4] set and the p orbitals from Huzinaga’s [5]. A scale factor of 1.414 wasusedfor the hydrogen atomic orbital to make it more ~.JJpropriate for the molecular enviromentS:. On the carbon, nitrogen and oxygen the s orbitaIs were contracted into three groups and the p orbitals into one for the SCF procedure; the hydrogen s’s were contracted into one group. The geometrical
parameters found experimentally by Coffey et al. [7] were employed for this calcul$ion (~CN= 1.49 A, ,-NO = 1.22 A, ,‘CH = 1.08 A, &cxo = 112.6O, &NCH = 109.00). 3. RESULTS
AND DISCUSSION
Results of the calculation are given in table 1. The eclipsed conformation is computed to be more stable than the staggered by 1.05 kcaI/ mole, and this is in excellent agreement with Coffey’s experimental findings [7]. The barrier is attractive do minant;
th2t is, the attractive
component of the total energy (Vn,) changes 3 The scaling factor has been extensively explored in
* Research supportedby the Chemistry Section of the National Science Foundation, Grant No. NSF-GP8907. ** NSF predoctoral fellow. 1966-1970.
numerous calculations at this Laboratoryfor rotational barriers and other properties. g particular, previous studies by Fink and Allen f61 find 1.414 to be the most appropriate choice for fhk scaIing factor in methanol, ethaneand hydrogenperoxide. In any case. the barrier is very insensitive to the scaie factor, as is shown on page 2274 of ref. [Sl. 75
Volume 5. number 2
Scaled
CHEMICAL PHYSICS LETTERS
energy
Table 1
components
the barrier effect is very small compared with the ordinary bonding which directly connects the
(au)
Staggered
atomsandthis particular overlap is dominated by the 10th molecular orbital (mainly nitrogen-
Eclipsed
Vnn
70.11864
70.14164
vee
123.02337
129.04364
T
166.63426
168.63594
V,, f Vet + T = Vr,,
367.77627
367.82123
- 536.41054
- 536.45717
- 168.63427
- 168.63594
vne = vatt ET
Changes in components going from staggered to eclipsed conformation AVatt = - 0.01663
traction between the unique hydrogen and the
oxygen lone pair lobes, principally contained in molecular orbital ‘7 and 12 (with overlap populations between 0 and H of 0.00763 and 0.01892, respectively). The attraction between the oxygen and hydrogen can be also thought of as a very weak hydrogen bond, since atomic populations in the methyl group are
= - 0.00167 Molecular orbital energies (au)
1 2 3 4 5 G 7 8 9 :1”
- 20.64339 - 15.77735 - 11.35298 1.53456 1.09534 0.87789 0.70879 0.67089 0.67050 0.5%X2 0.55295
- 29.64401 - 15.77779 - 11.35309 1.53464 1.39526 0.87854 0.71172 0.66935 0.66861 0.59058 0.55460
12
-
-
CL42020
0.425oti
Atomic populations C Hl H2 H3 N 0
G-7171 0.6902 O.G850 0.6850 i.0053 8.2174
more than the repulsive component
C Hl H2 H3 N 0 ( Vm+
6.7164 0.6912 0.6848 0.6848 7.0059 8.2169 V,,
4. CONCLUSIONS One finds that LCAO MO SCF calculations with the basis set employed here give the nitrosomethane rotational barrier in excellent agreement with experiment. Calculations from this and other laboratories with similar quality basis sets provide the correct ordering of barrier magnitudes in acetaldehyde, ethane, methanol, methyl amine, hydrogen peroxide, hydrazine and ethyl fluoride [2]. Thus, we have here still further evidence that the origin of rotational barriers is to be found within the framework of the Hartree-Fock approximation. ACKNOWLEDGEMENT
The computational assistance of Mr. William
Jorgensen
is appreciated.
+
rotates from the staggered to the eclipsed conformation. It is worth noting that the simple nuclear-nuclear repulsion model predicts a staggered geometry to be the lowest in energy. In a general way, attractive dominance can be viewed as very weak covalent bond formation. A T) when nitrosomethane
further detailed analysis of the barrier mechanism in this molecule may be cbtained by can-
sidering the Mluliiken overlap population between the oxygen and the hydrogen. eclipsed with it.
The total hydrogen-oxygen overlap population is negative (- 0.00598), in contrast to what one might expect. However, one must realize that
‘76
oxygen bonding with an oxygen-hydrogen overlap of - 0.03659). The major effect giving rise to the attractive dominance of the barrier is the at-
show that the hydrogens significantly positive.
AVrep = + 0.04496 f=T
1 March 1970
REFERENCES (11 J. Lowe, in: Progress in physical organic chemistry, vol. 6. eds. A. Streitweiser Jr. and R. W. Taft (Interscience, New York, 1968) p. 1; E. B. Wilson Jr. , in: Advances in chemical physics, vol. II. ed.1. Prigogine (Interscience. New York, 1959) p.67; Proc.Natl.Acad.Sci. (US) 43 (1957) 816. [Z] L.C.Allen, Chem.Phys.Letters 2 (1968) 597.
[3] R.B.Davidson and L-C-Allen. J.Chem.Phys..
submitted for publication.
[4] J.L.Whitten,
J.Chem.Phys.
44 (1966)
359.
[5] S.Huzinaga, J.Chem.Phys. 42 (1965) 1293.
[6] Fink and Allen, J.Chem.Phys. 46 (1967) 2261. [7] D. Coffey, C. 0. Britt and J.Boggs, J. Chem. Phys. 49 (1968) 591.
THE
ATTRACTIVE
CHEMICALPHYSICSLETTE_RS
ROTATIONAL
BARRIER
IN
I March 19?0
NLTROSOMETHANE
*
PETER A. KOLLMAN** and LELAND CoALLEN Depurfnlenf of Chemistry, Princeton University, Princeton, New Jersey 08540, U&l Received 9 December 1969 An ab initio LCAQ MO SCF calculation has been carried out on staggered and eclipsed forms of nitrosomethane. The rotational barrier found (1.05 kc&mole) is in excellent agreement with the experimental value (1.10 k&/mole). An energy component analysis shows nltrosomethane to be an attractive dominant barrier, thereby unequivocallydemonstratingthe possibility of a small scale, attractive covalent force as the controlling effect in some three fold barriers. 1. INTRODUCTION
Rotational barriers in molecuIes have been of interest to theoreticians for many years [l]. More recently, the ability to carry out high quality molecular orbital calculations has allowed one to gain a more detailed picture of the electronic changes in the molecule during internal rotation. The concepts of "attractive dominance” and %epulSive dominance” have allowed one to relate the detailed energy components of the molecular orbital calclllation to the modellike chemical concepts of attraction and repulsion [2]. Examples of three-fold barriers possessing repulsive dominance are ethane, methyl a&ne and methanol, while attractive dominance is found in acetaidehyde [3] and nitrosomethane. In both acetaldehyde (CH3-CHO) and nitrosomethane (CH3 -N-O), the oxygen is eclupsed to a methyl hydrogen. In acetaldehyde, the carbonyl hydrogen is eclipsed with respect to the methyl rotor; thus, one cannot determine whether the barrier is dominated by the eclipsing of the oxygen to a methyl hydrogen or by the staggering of the carbonyl hydrogen with respect to the methyl hydrogens. In nitrosomethane, the equilibrium geometry is determined by the attraction (or repulsion) between the nitroso oxygen and the methyl hydrogen&
2. COMPUTATIONAL METRY
METHOD
AND GEO-
A finite expansion molecular orbital calculation (LCAO MO SCF) was carried out using essentially Hartree-Fock quality atomic orbitals as a basis set. The s orbitals were taken from Whitten’s [4] set and the p orbitals from Huzinaga’s [5]. A scale factor of 1.414 wasusedfor the hydrogen atomic orbital to make it more ~.JJpropriate for the molecular enviromentS:. On the carbon, nitrogen and oxygen the s orbitaIs were contracted into three groups and the p orbitals into one for the SCF procedure; the hydrogen s’s were contracted into one group. The geometrical
parameters found experimentally by Coffey et al. [7] were employed for this calcul$ion (~CN= 1.49 A, ,-NO = 1.22 A, ,‘CH = 1.08 A, &cxo = 112.6O, &NCH = 109.00). 3. RESULTS
AND DISCUSSION
Results of the calculation are given in table 1. The eclipsed conformation is computed to be more stable than the staggered by 1.05 kcaI/ mole, and this is in excellent agreement with Coffey’s experimental findings [7]. The barrier is attractive do minant;
th2t is, the attractive
component of the total energy (Vn,) changes 3 The scaling factor has been extensively explored in
* Research supportedby the Chemistry Section of the National Science Foundation, Grant No. NSF-GP8907. ** NSF predoctoral fellow. 1966-1970.
numerous calculations at this Laboratoryfor rotational barriers and other properties. g particular, previous studies by Fink and Allen f61 find 1.414 to be the most appropriate choice for fhk scaIing factor in methanol, ethaneand hydrogenperoxide. In any case. the barrier is very insensitive to the scaie factor, as is shown on page 2274 of ref. [Sl. 75
Volume 5. number 2
Scaled
CHEMICAL PHYSICS LETTERS
energy
Table 1
components
the barrier effect is very small compared with the ordinary bonding which directly connects the
(au)
Staggered
atomsandthis particular overlap is dominated by the 10th molecular orbital (mainly nitrogen-
Eclipsed
Vnn
70.11864
70.14164
vee
123.02337
129.04364
T
166.63426
168.63594
V,, f Vet + T = Vr,,
367.77627
367.82123
- 536.41054
- 536.45717
- 168.63427
- 168.63594
vne = vatt ET
Changes in components going from staggered to eclipsed conformation AVatt = - 0.01663
traction between the unique hydrogen and the
oxygen lone pair lobes, principally contained in molecular orbital ‘7 and 12 (with overlap populations between 0 and H of 0.00763 and 0.01892, respectively). The attraction between the oxygen and hydrogen can be also thought of as a very weak hydrogen bond, since atomic populations in the methyl group are
= - 0.00167 Molecular orbital energies (au)
1 2 3 4 5 G 7 8 9 :1”
- 20.64339 - 15.77735 - 11.35298 1.53456 1.09534 0.87789 0.70879 0.67089 0.67050 0.5%X2 0.55295
- 29.64401 - 15.77779 - 11.35309 1.53464 1.39526 0.87854 0.71172 0.66935 0.66861 0.59058 0.55460
12
-
-
CL42020
0.425oti
Atomic populations C Hl H2 H3 N 0
G-7171 0.6902 O.G850 0.6850 i.0053 8.2174
more than the repulsive component
C Hl H2 H3 N 0 ( Vm+
6.7164 0.6912 0.6848 0.6848 7.0059 8.2169 V,,
4. CONCLUSIONS One finds that LCAO MO SCF calculations with the basis set employed here give the nitrosomethane rotational barrier in excellent agreement with experiment. Calculations from this and other laboratories with similar quality basis sets provide the correct ordering of barrier magnitudes in acetaldehyde, ethane, methanol, methyl amine, hydrogen peroxide, hydrazine and ethyl fluoride [2]. Thus, we have here still further evidence that the origin of rotational barriers is to be found within the framework of the Hartree-Fock approximation. ACKNOWLEDGEMENT
The computational assistance of Mr. William
Jorgensen
is appreciated.
+
rotates from the staggered to the eclipsed conformation. It is worth noting that the simple nuclear-nuclear repulsion model predicts a staggered geometry to be the lowest in energy. In a general way, attractive dominance can be viewed as very weak covalent bond formation. A T) when nitrosomethane
further detailed analysis of the barrier mechanism in this molecule may be cbtained by can-
sidering the Mluliiken overlap population between the oxygen and the hydrogen. eclipsed with it.
The total hydrogen-oxygen overlap population is negative (- 0.00598), in contrast to what one might expect. However, one must realize that
‘76
oxygen bonding with an oxygen-hydrogen overlap of - 0.03659). The major effect giving rise to the attractive dominance of the barrier is the at-
show that the hydrogens significantly positive.
AVrep = + 0.04496 f=T
1 March 1970
REFERENCES (11 J. Lowe, in: Progress in physical organic chemistry, vol. 6. eds. A. Streitweiser Jr. and R. W. Taft (Interscience, New York, 1968) p. 1; E. B. Wilson Jr. , in: Advances in chemical physics, vol. II. ed.1. Prigogine (Interscience. New York, 1959) p.67; Proc.Natl.Acad.Sci. (US) 43 (1957) 816. [Z] L.C.Allen, Chem.Phys.Letters 2 (1968) 597.
[3] R.B.Davidson and L-C-Allen. J.Chem.Phys..
submitted for publication.
[4] J.L.Whitten,
J.Chem.Phys.
44 (1966)
359.
[5] S.Huzinaga, J.Chem.Phys. 42 (1965) 1293.
[6] Fink and Allen, J.Chem.Phys. 46 (1967) 2261. [7] D. Coffey, C. 0. Britt and J.Boggs, J. Chem. Phys. 49 (1968) 591.