J. Phys. Chem. 1994, 98, 1856-1863
1856
Effect of an Adding Radical’s Electronegativity on the Geometries and Formation Energies of Nitroso Spin Adducts Susan L. Boyd’*+and Russell J. Boyd$ Department of Chemistry, Mount Saint Vincent University, Halifax, Nova Scotia, B3M 2J6, Canada: and Department of Chemistry, Dalhousie University. Halifax, Nova Scotia, B3H 4J3, Canada Received: November 23, 1993’
Ab initio calculations on the nonplanarity of adducts with spin traps H-N=O,
CH3-N-O, and CH3CHz-N=O indicate that the degree of nonplanarity at the N correlates weakly with the electronegativity of the adding radical. The degree of nonplanarity depends on the level of theory used: extending the basis set beyond the 6-3 1G(d) level within the HartreeFock framework reduces the nonplanarity slightly, and including the effects of electron correlation at the MP2/6-31G(d,p) level reduces it considerably more. However, the trends within a given theory level and basis set are essentially the same. Formation energies indicate that in general the adducts are strongly favored over the free radical and the spin trap.
Introduction Electron spin resonance (esr) allowsdetection and identification of relatively stable free radicals. For short-lived radicals, one method used over the past 25 years’ for generating a detectable steady-state concentration is “esr spin trapping”, where the unstable radical adds to an unsaturated compound known as the “spin trap”. The product of this addition is a more stable radical, the “spin adduct”,which can then be detectedvia esr. Two classes of spin traps have been used extensively, the C-nitroso compounds, R-N-O, and the nitrones, R-CH-NR’O; in general the nitroso function has been applied in chemical systems while in biological systems, because the nitroso compounds are toxic, the nitrones have been more often used. This study focuses on the nitroso spin traps. +
R-N=O
nlroso spin trap
-
%’
R-N(0)-R’
adding radical
spin addud
In particular, in connection with our interest in properties which correlate with electronegativity,2Jwe have undertakenan ab initio study of the suggestion that when atoms (or groups) which are more electronegative than the aminoxyl nitrogen are added to the nitroso spin traps, there is greater nonplanarity about the nitrogen atom of the aminoxyl function.’ (This proposal is a natural extension of the generalization that L BAB decreases in compounds as B becomes more electronegative: which has been tested for some amines.5.6 In explanationof the trend in amines, it has been argued that lone pairs on a more electronegative substituent have a repulsive interaction with the lone pair on the central atom.6) A comment on our choice of a prototype system is necessary. All C-nitroso compounds are very reactive, which is the reason why they are useful as spin traps. In addition to trapping, however, they can participate in other competing reactions. For example, they may undergo thermal or photochemical dissociation,7 R-N-0 dimerization? 2R-N
II
0
-
+ ‘NO
R’
4 R-N=N--R
I
I
‘0 0’
or reaction with olefins via an “ene” mechani~m.~ Mount Saint Vincent University. t Dalhousie University. Abstract published in Advance ACS Abstracts, January 15, 1994.
0022-3654/94/2098-l856$04.50/0
R-N=O
+
C=C-C
-
R-N-C-CzC
I
OH
If the R-group is a sterically hindered aliphatic group or an aromatic group, the reactivity toward these competing reactions is reduced, and the compound is more selective in its reactivity with free radicals. Hence, the most frequently used nitroso spin traps are derivatives of tertiary alkyl or aromatic hydrocarbons, e.g., 2-methyl-2-nitrosopropane, (CH3)3C-N=O, and nitrosobenzene, C~HS-N=O. These are, however, too large to be treated by ab initio methods with reasonably large basis sets (i.e., split valence, polarized and/or diffuse functions). Consequently, the simple unsubstituted nitroso hydride ( H - N 4 ) and the smallest organic nitroso compounds, nitrosomethane (CHs-N=O) and nitrosoethane(CH~CHZ-N-O), have been used in this study as prototypes for the more highly substituted nitroso spin traps. While these small nitroso compounds are too reactive for experimental use, they do give a picture from which generalizations can be made about the larger compounds.
Computational Methods Calculationswere performed at the unrestrictedHartree-Fock (HF) and second-order M~rllerPlesset (MP2) levels using the GAUSSIAN 9010computer program. Standard 6-31G(d) basis sets” were used for most geometry optimizations;this basis set is known to accurately predict the angles at nitrogen atoms12J3 because, being split valence and having d-polarization functions on C and N, it provides a balanced description of the localization and delocalization tendencies of the lone pair on the N.14 Single point energies were determined at the MP2/6-31G(d,p)//HF/ 6-31G(d) level. Some optimizations were also done using the STO-3G, 3-21G, 6-3lG+(d), 6-3lG(d,p), and 6-31G+(d,p) basis sets and at the MP2/6-3lG(d,p)//HF/6-3lG(d) (or MP3 or MP4, with the same basis set) levels for comparison. When appropriate, the existence of multiple stable rotamers was investigated; the global minimum-energy structures are given except where otherwise noted. All optimized structures were checked by vibrational frequency analysis at the 6-31G(d) level to confirm that a minimum energy conformation had been located. Spin and charge densities were obtained by Mulliken population analyses. Results and Discussion
Nonplanarity at N in amines and amides can be expressed by three methods. Firstly the sum of the angles at N can be used: this will be 360’ for a planar N and 328.4’ for a tetrahedral 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1857
Nitroso Spin Adducts
-
TABLE 1: Nonplanarity of HNO Spin Adducts at the N (HF/6-31C(d) Opthized Geometries) R*
+
H-N=O
H-N-0.
I
R
spin densityd Xb
CUc (deg)
dipole moment (D)
'F 'OOH
4.00 3.59
*OH "=C "4 'NH2
3.55 3.26 3.12 3.12
'C1 'CF3 *CiN
3.01 2.71 2.69 2.66 2.65
58.0 49.4 54.7 53.0 40.7 0.0 37.4 37.7 47.8 34.0 0.0 0.1 23.5 28.0 34.5 0.0 32.7 32.3 0.0 0.0 0.0 0.0
2.37 3.57 3.53 2.00 2.85 2.09 1.46 4.04c 2.17 2.13 3.46 2.49 2.48 2.30 2.56 2.69 2.92 2.90 1.61 1.92 2.38 4.46
'RQ
H " 'SH
2.60 2.58 2.55 2.55 2.17 1.92 1.90 1 .oo
'CH2F 'C2H3 'CH, GHs 'PH2 'BH2
'SiH, 'Li averagd
N
0
0.21 0.17 0.17 0.22 0.21 0.13 0.29 0.25 0.22 0.18 0.20 0.26 0.25 0.25 0.21 0.35 0.30 0.31 0.25 0.07 0.24 0.78 0.24
0.75 0.82 0.8 1 0.76 0.79 0.75 0.73 0.77 0.76 0.83 0.81 0.75 0.79 0.78 0.81 0.65 0.73 0.72 0.78 0.87 0.79 0.37 0.77
* 0.05
charge densityd 0
N +0.18 +0.06 +0.04 +0.22 -0.09 -0.07 -0.13 -0.15 -0.29 -0.41 -0.39 -0.46 -0.49 -0.40 -0.33 -0.27 -0.28 -0.28 -0.46 -0.33 -0.47 -0.37
* 0.04
-0.26 -0.24 -0.24 -0.28 -0.24 -0.26 -0.33 -0.29 -0.24 -0.25 -0.23 -0.28 -0.27 -0.27 -0.28 -0.36 -0.33 -0.34 -0.28 -0.25 -0.29 -0.59 -0.28
* 0.04
0 In cases where more than one set of values is presented, the additional rotamers listed have significantly different geometries at the N. The first rotamer listed in each instance is the global minimum energy structure at the HF/6-31G(d) level, and others are local minima, as confirmed by the frequency analysis. b Electronegativity values are from, or were calculated by the method of, ref 3. e The deviation from planarity, a,is calculated according to eq 1. d Mulliken population analysis. CA small change in the position of the H atoms can have a dramatic effect on the dipole moment. /The average excludes the outliers, 'BH2 and 'Li.
one.l4J5 Secondly,the pyramidalization angle which is the angle between the extension of one N-substituent bond and the plane formed by the N and its other two substituents can be used:6J6 this is 0' for planar N and 54.7' for tetrahedral N. We have chosen to measure the nonplanarity by a third method which involves an angle defined by
= 180' - (R,-N(O)-RJ
(1) where Rl-N(O)-R, is the dihedral angle formed between the two substituents on N as viewed along the N - O bond. Thus, planar N gives a = Oo, while tetrahedral N gives a = 60'. This methodgives thelargest rangeofangleswithin thevariousmethods for indicating nonplanarity; however, the method used clearly does not affect our conclusions. The results of the HF/6-3 1G(d) study of the nonplanarity (a) of nitroso hydride adducts, H-N(0)-R, can be seen in Table 1, and also in Figure 1. There is a correlation between a and electronegativityfor those species which do give nonplanar adducts (r = 0.956), but when all adducts are considered the correlation is much poorer (r = 0.751). Many radicals actually produce planar adducts. These include most of the unsaturated radicals, e.g., "4, * G N , *CH==CH2, and * W H , with a notable exception being * N ewhich yields a nonplanar adduct. Also radicals of relatively low electronegativity, e.g., 'PHI, 'BH2, 'SiHp, and *Li produce planar adducts. A similarly weak correlation between electronegativity and nonplanarity in substituted amines has been reporteda6 There is little variation in the spin density (Table 1) among the adducts irrespective of whether the adduct is planar or not: the N on average has about 24 f 5% and the 0 about 77 f 4% of the spin density. (The only adducts which deviate significantly are the 'BH2 and 'Li species; in the *BH2adduct the spin density is more strongly associated with the oxygen (87%) whereas in the 'Li adduct, it is primarily associated with the nitrogen (78%).) The charge density is fairly constant for the 0 throughout the series (-0.28f 0.04)but varies for the N depending on the relative electronegativity of the adding radical. Thus, resonance contributor A is favored, but B must also be significant to account CY
60
C
50
0
40 m
-m
30
E
$
.-cg m g
20 10
0 1
0
2
3
Electronegativity,
4
5
x
Figure 1. Deviation from planarity, a,for adducts of H-N=O as a function of the electronegativityof the adding radical. For the nonplanar adducts (a> 0) the correlation coefficient is 0.956 (-); for all adducts, r = 0.751 (- -).
-
for the spin and charge densities and for the high dipole moments observed for the adducts. H-N->-
.. I
A
.+
**
H-N-O: I
..0'
B
The adducts of nitrosomethane (Table 2 and Figure 2) give similar results. Again, an increase in electronegativity results generally in an increase in a (r = 0.857 for all nonplanar adducts, but r = 0.753 when all adducts are considered). The a-values are somewhat lower for the C H 3 - N 4 adducts than for the H-N=O ones; for example, the *Fadduct with nitroso hydride has a = 58.0' while that with nitrosomethane has a = 52.0'. AS with the H-N=O adducts, CHs-N=O adducts are planar with the radicals " 4 , *BH2, and 'Li; the lower energy conformer of the adduct with 'CH=CH2 is also planar. But,
Boyd and Boyd
1858 The Journal ofPhysica1 Chemistry, Vol. 98, No. 7, 1994
TABLE 2:
-
Nonplanarity 01 CH3N0 Spin Adducts at the N (HF/6-31G(d) Optimized Geometries) R'
+
CH3-N=0
CH3-N-O'
k spin density
R' 'F
'OOH 'OH "=C
"4 "HI
'CI
X'
'SH 'CHIF 'CIH3
2.60 2.58
'CHI 'CIHS
2.55 2.55
'PHI
2.17 1.92 1.90 1.00
H "
'BHI
dipole moment (D)
52.0
3.09 4.13 3.85 1.57 3.57 2.93 1.57 4.19 2.84 4.12 2.82 2.75 2.63 2.61 2.79 3.17 2.98 2.86 3.00 3.24 2.20 2.52 4.48
40.0
50.6 43.4 34.7 0.0 33.5 34.7 40.4 11.7
3.55 3.26 3.12 3.12 3.01 2.69 2.66 2.65
*C=N
a (deg)
4.00 3.59
10.1
23.6 25.0 30.6 0.0 5.2 30.4 30.7 26.4 15.3 0.0 3.7 0.0
N
0
0.29 0.24 0.22 0.43 0.27 0.13 0.35 0.29 0.29 0.23 0.29 0.28 0.29 0.26 0.38 0.40 0.35 0.35 0.37 0.29 0.08 0.28 0.82 0.30 f 0.07
0.68 0.76 0.76 0.56 0.73 0.74 0.67 0.73 0.70 0.78 0.71 0.75 0.74 0.76 0.62 0.57 0.68 0.67 0.66 0.74 0.85 0.74 0.30 0.70 f 0.06
'SiH3 'Li averageb The average excludes the outliers, 'BH2 and 'Li. L1 See footnotes for Table I. 60
,
/,'
I
charge density N
0
+0.33 +0.19 +0.16 +0.23 +0.02 +0.03 0.0 4.01 4.15 4.26 4.32 4.26 4.27 4.19 4.13 4.10 4.13 4.14 4.13 4.31 4.19 4.34 4.19
4.30 4.28 4.27 4.41
4.28 4.29 4.37 4.32 4.28 4.27 4.33 4.29 4.30 4.32 4.40 4.43 4.37 4.37 4.38 4.31 4.26 4.32 4.62 4 . 3 2 f 0.05
a h
01
0
1
2
4
3
Electronegalivity,
= 39.9"
5
x
Figure 2. Deviation from planarity, a. for adducts of C H r N = O as afunctionoftheelectronegativityoftheaddingradicdl. For thenanplanar adducts (a> 0) the correlation wefficient is 0.857 (-); for all adducts, I = 0.753 (-. -).
within thesetofaddingradicalsconsidered,therearefewerplanar adducts with nitrosomethane than with nitroso hydride. For example, whereas the C=N adduct of HNO is a planar species, itsadductwithCH3NO hassignificant nonplanarity (a= 11.7'); similarly, the 'C=CH, 'PHI, and 'SiH3 adducts with C H 3 N 0 arenonplanar (a= 10.1". 15.3°,and3.10,respectively)incontrast with their counterparts with HNO, one of the higher energy conformers of ' C I H ~with nitrosomethane is also nonplanar (a = 5.2O). Since delocalization of the lone pair on the N is less likely for H-N(0)-R than for CHI-N(0)-R, the fact that the former are planar more often than the latter is surprising. Again there is little variation in the spin densities throughout the CH3-N=0 adducts; N on average has 30 f 7% and 0 has 70 6%. which indicates that slightly more spin is associated with the N and less with the 0 than in corresponding H-N=O adducts. The 'BH2 and 'Li adducts are the only species with significantly different spin densities. The charge densities on 0 are marginally more negative for the CH3-N=O series (4.32 0.05) than for the H-N=O one. The dipole moments are high.
*
*
we 3. HF/6-31G(d) aptimi; - N 4 and (b) CHa-N=O
Adducts of H'with HNO and CH3N0 are not included in Table 1 or 2, or in Figure 1 or 2, because the electronegativity ofH isnotavailahlehy themethod~sed;~ however, the'Hadducts are nonplanar, as can he seen in Figure 3. Indeed, in comparison to the other adducts considered, the 'H adducts are considerably more nonplanar than the Pauling electronegativity for H (x = 2. I)" would suggest: the 'expected" a as estimated from Figure 1 or 2 is under 20". whereas the 'H adduct has a = 39.9' and 32.7' with H-N=O and CH3-N=O, respectively. The data of Tables 1 and 2 show that variation of planarity at N in nitroso spin adducts is to some extent a steric effect: different rotamers of a given adduct often have different degrees of nonplanarity. For example, the lowest energy conformer of theperoxyladductofH-N=Ohasa=49.4°(Table ]),whereas
The Journal of Physical Chemistry, Vol. 98, No. 7, I994
Nitroso Spin Adducts
TABLE 3
1859
NO, NH, and NC Bond Lengths and Double Bond Character in H-N-0
and CHa-N-0
R
R
I
I
'R
'F 'OOH 'OH ' N e ON Q 'NH2 'C1 'CF3 *C=N H " 'SH 'CH2F 'CH4H2 'CH3 'CH2CHp 'H 'PH2 'BH2 'SiH3 'Li av bond length av double bond characteP
1.232 1.258 1.253 1.261 1.242 1.260 1.248 1.275 1.273 1.267 1.267 1.278 1.254 1.267 1.267 1.Z76 1.276 1.294 1.286 1.312
1.005 1.004 1.002 1.000 1.007 0.999 1.004 1.001 0.998 0.998 1.ooo 1.001 1.001 1.001 1.002 1.ooo 1.ooo 1.ooo 1.001 1.007
1.223 1.249 1.236 1.255 1.248 1.256 1.239
1.456 1.454 1.450 1.457 1.462 1.449 1.46 1
1.27 1 1.265 1.265 1.273 1.258 1.263 1.263 1.267 1.276 1.294 1.282 1.314
1.458 1.454 1.457 1.452 1.451 1.448 1.449 1.445 1.453 1.454 1.452 1.440
1.267 & 0.018
1.002 & 0.003
1.263 & 0.021
1.453 & 0.005
0
59.6%
6 1.4%
0
The percent double bond character is defined by eq 2 in the text. All optimizations were done with the 6-3 1G(d) basis set. Reference bond lengths are as follows: the NO bond length in H2N-OH is 1.403 A; in HN = 0 it is 1.175 A; the NH bond in NH3 is 1.003 A; the NC bond in HzN-CHa is 1.453 A. 0
one of its higher energy conformershas a = 54.7'. (In each case, the rotamer is a local energy minimum, with no imaginary frequencies.) In the HOO' adduct with CH3-N=O, there is an even greater variation of nonplanarity with the orientation of theperoxylportion: a = 40.0°-50.6' (Table2). Rotationwithin several other adducts also produces changes in planarity: the 'SH adduct, and to a smaller extent the 'NH2 adduct, have rotamers with different a. Nor is it only polar radicals that have adducts with rotamers of varying a: the * C ~ Hadduct S with CH3-N=0 (Le., CH~-N(O)-C~HS) has a varying from 30.7' to 26.4' depending on the dihedral angle about the newly formed NC bond. Yabushita and Gordon6considered the nonplanarity of substituted amines and observed in particular that the cyanosubstituted speciesis more planar than its electronegativity would suggest. They felt that this is due to stabilization of the planar form by ?r-interaction between the cyano group and the amino nitrogen, i.e., that thereis somedoublebond character. Similarly, BH2and C H 4 produce deviation from the correlationbetween group electronegativityand isodesmic stabilization energies for substituted ketenesI8 and dia~omethanes.1~ This was attributed to ?r-donation from the ketene or diazomethane mystem to the given group, i.e., to resonance stabilization, with a major contributor having a double bond between the group of interest and the ketene or diazomethane. To investigate whether a similar phenomenon was occurring in the nitroso spin adducts,the "double bond characters" of the three bonds to N within the adducts were considered. The percent double bond character (%DBC, eq 2) was estimated by comparing the optimized HF/6-3 1G(d) NX (X = H, 0, or R) bond length in the adduct with single and double NX bond lengths in related reference H2N-X and HN=X molecules, whose geometries were also optimized at the HF/6-31G(d) level.
[
N-Xref
molecule - N-Xadduct
]
x 100% (2) = Xref molecule Firstly, there is no significant variation in the NO bond length with R in the H-N(R)-0 or CH3-N(R)-O series (Table %DBC =
N-Xref molecule-- N
3); for example, the average NO bond length in the adducts of H-N=O is 1.267 f 0.019A. (This is about 60% double bond character compared to the bonds in H2N-OH (1.403 A) and HN=O (1.175 A).) Secondly, both the NH bond lengths in H-N(R)-0 (1.002 f 0.003 A) and the NC bond lengths in H3C-N(R)-0 (1.453 f 0.005 A) are independent of R and show no double bond character. However, the third bond to the adduct N, N-R, does show avariation in doublebond character which dependson R. Results are presented in Table 4. In general, for the H-N=O series, those radicals with electronegativities above 2.5 which give nonplanar adducts (a > 0) have relatively low double bond character, ranging from -12% for the 'CF3 adduct (i.e., the NC bond is actually longer in the adduct, HN(O)-CFs, than in the analogous amine, H2N-CF3) to +18% for the HOO' adduct, HN(0)-OOH (here the NO bond is intermediate between H2NOOH and H N 4 ) . On the other hand, species in this electronegativityrange (above 2.5) which yield planar adducts (a = 0) have considerably greater double bond character: the range in this case is 34% (in the ' N = O adduct) to 53% (in the TECH adduct). The *N=C adduct is an obvious exception to this generalization: its adduct is nonplanar (a> 0) yet it exhibits considerable double bond character (about 33%). The radicals of low electronegativity ('PHI, 'BH2, 'SiH3, and 'Li) give negative double bond character (their adducts have longer NR bonds than are found in the related amino compounds) and these are all planar. Whether the double bond character of the N-R bonds in H-N=O adducts has any significant correlation with nonplanarity, however, is questionable in view of the fact that the comparable N-R bonds in the CH3-N=O adducts have very similar double bond character, yet in many cases are nonplanar when the H-N=O adducts are planar. A few adducts with nitrosoethane (CH&Hz-N=O) were also optimized at the HF/6-3 1G(d) level to ascertainif the bulkier alkyl group would affect the nonplanarity. Their deviations from planarity at the N are present in Table 5. The geometries at N are essentially identical to those for the corresponding adducts
1860 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
TABLE 4
NR Bond Lengths and Double Bond Character in and CHI-N-0
H-N-0
I
I
R
bond lengths used for comparison
I
I
N-R
R
R
length (A)
R
CHs-N-0
H-N-0
R'
D B C (96).
length (A)
D B C (96)
'F 'OOH 'OH C I"
1.370 1.350 1.376 1.343
18 12 33
1.379 1.356 1.379 1.346
15 11 32
"4
1.327
34
1.329
33
'NH2 'C1 'CF3 *C=N
1.39 1 1.728 1.414 1.344
11
1.397 1.741
8
-12 54
1.349
52
"H
1.349
53
'SH 'CH2F 'CHECH2
1.698 1.418 1.378
7 -3 38
1.710 1.422 1.382
'CH3 'C2H5 'H 'PH2 'BH2
1.442 1.449 1.ooo 1.727 1.411 1.758
5 3
1.448 1.457
-14 -13 -22
1.737 1.414 1.764 1.836
'SiH3
'Li
Boyd and Boyd
0 -5 36 6 2 0 -20 -1 5 -26
N=R
in
bond length (A)
NH2F HzN-OOH H2N-OH HzN-NHCH3 H2N-NEC H2N-NHOH H2N-N4 H2N-NH2 HZN-Cl H2N-CF3 HzN-CHzNHz HzN-CHzNHz HzN-CHzCH, HzN-hCH H2N-SH HzN-CHzF HzN-CHzCH, HzN-CHCHz H~N-CHB HzN-CzHs H2N-H HzN-PHz HzN-BHz HzN-SiH, HZN-Li
1.385 1.388 1.403 1.407 1.356 1.390 1.438 1.413 1.731 1.394 1.449 1.344 1.456 1.357 1.710 1.412 1.456 1.393 1.453 1.456 1.003 1.706 1.389 1.725 1.750
in
bond length (A)
HN=O HN=O HN=NCH3
1.175 1.175 1.215
HN=NOH
1.203
HN=NH
1.216
HN=CF2 HN4HNH2
1.225 1.255
HN=CHCH3
1.253
HN=S HN-CHF HN4HCH3 HN=C=CH2 HN=CH2 HN4HCH3
1.539 1.232 1.253 1.213 1.250 1.253
HN=PH HN-BH HNSiH2
1.554 1.223 1.573
a The percent double bond character (96DBC) is estimated from the single bond lengths, N-R, and double bond lengths, N=R, in the reference molecules listed, according to eq 2. All structures, including reference molecules, were optimized with the 6-31G(d) basis set.
TABLE 5 Nonplanarity of CH&€I#O Adducts at the N (HF/6-31G(d) Optimized Structures) adding radical, 'R adduct a (deal 'F 'OH 'C1 'C=N 'H
CH~CHZ-N(O)-F CH3CH2-N(O)-OH CH,CHz-N(O)-Cl CH$Hz-N(O)-C=N CH~CHZ-N(O)-H
51.7 43.8 40.4 11.3 32.3
with nitrosomethane, showing that the extra methyl group has essentially no effect. Nonplanarity at the N atom has long been recognized to be overestimated by the STO-3G basis set, and underestimated by the3-21Gand4-1Gsplitvalence basissets;l2furthermore,studies on methylamine, and particularly on formamide, demonstrate the sensitivity of the angles at N to the basis set chosen,*Owith large and balanced basis sets being required. Since the 6-3 1G(d) basis set has been considered to give fairly accurate results, this basis set was chosen for our investigation.12 But to assess whether the planarity at the N for nitroso spin adducts is basis-set sensitive as it is for amines and amides, and more specifically whether the correlationbetween nonplanarity and electronegativityis different with different basis sets, we have optimized several adducts using a variety of basis sets, including polarization functions and diffuse functions, and also using second-orderMprller Plesset perturbation theory the 6-31G(d,p) basis set. The results are presented in Tables 6 and 7 and in Figures 4 and 5. With only one very minor exception (CH3-N(O)-F), the STO-3G basis set indeed gives the highest a. As expected, the split valence 3-21G basis set causes a to drop in virtually every example, but not by as large an amount as has been observed for the amines.12 The CY drops significantly further for the 6-3 lG(d) basis set. Introduction of more polarization functions (6-31G(d,p)), diffuse functions (6-31+G(d)), or both (6-31+G(d,p)), causes additional small decreases in a. Finally, optimization at the MP2/6-31G(d,p) level results in the lowest a-values among the various levels of theory and basis sets considered. Like the HF/6-31G(d) optimized geometries for the H-N=O
adducts (Table l), the optimized geometries at the various levels considered (Table 6) show increases in nonplanarity as the electronegativityof the adding radical increases. But the increase is certainly not monotonic, as can be seen in Figure 4: the *C=N adduct is planar at all levels, the 'C=C-H adduct is planar at all levels but STO-3G, and the 'SH adduct is nearly planar at the MP2/6-31G(d,p) level. R-values range from 0.501 for the STO-3G level steadily increasing to 0.840 for the MP2/6-3 1G(d,p) structures. Therefore, even at the highest level considered, for this set of adducts the correlation between electronegativity and nonplanarity is, at best, weak. The CHp-N=O series yields very similar results (Table 7, Figure 5): the larger the basis set and/or the higher the level of theory, the smaller the calculated deviation from planarity at the N for the spin adducts. Moreover, within any given level, the correlation between nonplanarity and electronegativity is weak and is approximately the same as for other levels and basis sets, varying from r = 0.749 to r = 0.817. The data in Tables 6 and 7 do not provide evidence that the angle has converged to a final value even with MP2 optimization. To assess if the angles have reached their limits in the smallest adducts, the 'H and 'F adducts of H - N 4 were optimized at the MP3 and MP4 levels, in addition to the levels already discussed. The results for all levels of H-N(0)-H and H-N(0)-F optimization are presented in Table 8. For the *Hadduct, a drops with enlargementof the basis set, as observed for the adducts discussed above; with the STO-3G basis, a = 62.2', while with the6-31+G(d,p) basis, a = 32.4. At the MP2/6-31G(d,p) level, a drops further to 12.8', but expansion to MP3 and MP4 level optimizations increases it to 23.8O and 24.6', respectively. Whether this indicates convergence is certainly not clear. For the 'F adduct, a again drops steadily from the STO-3G level value of 67.6' to the 6-3 l+G(d,p) value of 56.8'. With electron correlation considered, it drops more, so that at the MP2/631G(d,p) level a is 50.5'; MP3 and MP4 optimizations give essentially the same degree of nonplanarity (51.7' and 53.4') suggesting that convergence has been reached for this species.
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1861
Nitroso Spin Adducts
TABLE 6 Effect of Level of Theory and Basis Set on Nonplanarity at N for HNO Spin Adducts. spin adduct STO-3G 3-21G 6-3 1G(d) 6-3lG(d,p) 6-3 1+G(d) 6-3 l+G(d,p) HN(0)OH energy -202.506 73 -204.055 41 -205.196 76 -205.206 69 -205.206 12 -205.216 25 ff
HN(0)NC energy ff
HN(0)Cl energy ff
HN(0)CN energy ff
HN(0)CCH energy ff
HN(0)SH energy ff
HN(0)CHs energy a
MP2/6-3 lG(d,p)
67.6
63.6
53.0
51.7
51.7
50.5
-205.729 61 42.1
-219.171 21 63.6
-220.796 27 50.5
-222.041 47 40.7
-222.045 41 38.8
-222.050 96 39.5
-222.054 84 37.5
-222.646 82 24.6
-582.665 86 67.1
-586.339 03 57.2
-589.248 90 47.8
-589.252 60 46.7
-589.254 73 46.6
-589.258 39 45.4
-589.727 18 41.9
-219.223 23 0.1
-220.863 97 0.0
-222.101 70 0.0
-222.105 96 0.0
-222.108 66 0.0
-222.1 12 78 0.0
-222.71 1 39 0.1
-203.407 74 50.5
-204.91 1 72 0.0
-206.047 87 0.1
-206.054 11 0.1
-206.056 50 0.0
-206.062 68 0.0
-206.635 12 0.0
-521.857 33 -521.859 79 49.9 55.4
-525.201 76 -525.201 76 30.5 30.5
-527.880 58 -527.880 55 23.5 28.0
-527.888 32 -527.888 27 20.4 24.9
-527.886 98 -527.886 96 21.1 25.6
-527.894 70 -527.894 66 18.0 21.9
-528.354 16 -528.354 16 0.8 4.1
-167.270 29 58.5
-168.482 52 37.3
-169.424 99 32.7
-169.433 48 31.2
-169.431 79 30.5
-169.440 19 28.6
-169.917 91 18.2
a Energies in atomic units, a in degrees. For the 'SH adduct with H-N=O, the two minimum energy structures are very similar in energy at the 6-31G(d) level, so both conformers were monitored at all levels. At the 3-21G level in fact only one minimum energy structure was found.
TABLE 7: Effect of Level of Theory and Basis Set on Nonplanarity at N for CHDO Spin Adducts. spin adduct STO-3G 3-21G 6-31G(d) 6-31G(d,p) 6-31+G(d) 6-31+G(d,p) CH,N(O)F enerw -264.703 19 -266.740 57 -268.216 90 -268.221 67 -268.228 52 -268.233 20 64.8
65.9
52.0
51.9
51.3
51.2
-268.883 62 48.8
-257.756 69 59.3
-259.616 96 51.3
-261.079 36 34.7
-261.084 11 34.7
-261.089 19 33.5
-261.093 89 33.2
-261.838 58 26.7
-621.251 28 61.8
-625.160 67 52.7
-628.286 48 40.4
-628.291 22 40.4
-628.292 36 39.7
-628.297 04 39.5
-628.920 72 39.9
-257.808 51 44.2
-259.681 80 10.3
-261.137 98 11.7
-261.142 75 11.7
-261.144 96 11.7
-261.149 68 11.6
-261.900 51 11.7
-241.989 96 45.9
-243.728 03 8.8
-245.082 87 10.1
-245.098 97 10.1
-245.091 41 10.0
-245.098 24 9.4
245.835 60 10.3
-205.852 58 53.8
-207.300 01 39.9
-208.459 99 30.4
-208.469 39 30.4
-208.466 69 29.1
-208.457 92 28.8
-209.106 29 24.5
-167.270 29 58.5
-168.482 52 37.3
-169.424 99 32.8
-169.433 48 31.2
-169.431 79 30.5
-169.440 19 28.6
-169.917 91 18.2
I I
ff
CH3N(O)Nd energy ff
CHsN(0)Cl energy ff
CHjN(O)-N energy ff
CHaN(0)mH energy a
CHoN(0)CHs energy ff
CHsN(0)H energy ff a
MP2/6-31G(d,p)
Energies in atomic units, a in degrees.
TABLE 8: Effect of Level of Theory and Basis Set on Nonplanarity at N for the 'H and 'F Adducts of HNO 'H adduct 'F adduct basis set energy(au) a energy(au) STO-3G 3-21G 6-31G(d) 6-31G(d,p) 6-31+G(d) 6-31G+(d,p) MP2/6-3 lG(d,p) MP3/6-3 lG(d,p)' MP4SDTQ/6-31G(d,p)
-128.688 -129.667 -130.389 -130.397 -130.396 -130.404 -130.730 -130.737 -130.751
20 00 81 50 65 49 90 35 54
62.2 35.2 39.9 36.2 37.0 32.4 12.8 23.8 24.6
-226.116 -227.916 -229.175 -229.179 -229.187 -229.191 -229.688 -229.683 -229.711
71 03 69 46 25 00 84 22 95
The pyramidal form is favored, but the inversion barrier (Le!, small, 0.4 Id/mol. The formation energies for the adducts, calculated as single point energiesat theMP2/6-31G(d,p)//HF/6-31G(d) level, and for some adducts, calculated as energies for structures optimized at the MP2/6-3 lG(d,p)//MP2/6-3lG(d,p) level, are given in Table 9. The AE values are similar for the H-N=O and CHs-N=O series and for the single point and optimized energies. In general, formation of the adducts from the unstable free radical and the spin trap is highly favored, yielding large negative AE values; the exceptionsare the adducts of 'OOH and "4, which are slightly exothermic or endothermic. The carbon radicals produce larger negative AE values than nitrogen, oxygen, sulfur, or halogen radicals; 'H, 'BH2, and 'Li adducts are also very stable, reflecting the high reactivity of the starting free radicals. Ep:pyraddal - Eplaear) is very
CY
67.6 66.0 58.0 57.1 57.9 56.8 50.5 51.7 53.4
a Atthislevelthe planar formof H2NOhasE=-130.73719au, which is higher in energy than the pyramidal form, unlike the results reported optimization (see ref 6). for the MP2/6-31G(d)//HF/3-21G(d) However, there is no guarantee that the MP expansion will in general converge for properties of interest.21 It is interesting to note that, while the planar form of H2NO is reported to be lower in energy than the pyramidal form at the MP3/6-3lG(d)//HF/3-2lG(d) level,6 at the MP3/6-31G(d,p)//MP3/6-3lG(d,p) level we do not find this to be the case.
Conclusions While there is a general tendency for the adducts of the spin traps H-N=O, CH3-N=0, and C H ~ C H Z - N ~to,become more nonplanar as the electronegativity of the adding radical increases, the correlation is at best weak. Adducts with radicals
1862 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
-
5-
0
0
h
i
.
-
-0
L L
C m
C m
-
-am
e
E
a E
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0
m
B
40
e
L
I
30
! 10
!
80
2 50
‘
3.00 Electronegativity,
3.50
’
-
20
0 ’ 2.00
Boyd and Boyd
1
0‘
4.00
2 00
2 50
x
3 00
Electronegativity,
I
3 50
4 00
3 50
4 00
3.50
4.00
3 50
4 00
x
i
-
0 -
0
h L L
m C m
-
a E
e C
P m
>
B 02 00
2 50
3 00 Electronegativity
80
e
70
0
i C m
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6o 50
40
c.
.2
-I
0
60
E.
t
h
m
-am
-
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e C
20
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-
W
n
I
70
L
30
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z
1
x
I
m
n
10 I
02 00
2 50
3 50
3 00 Electronegativity
2 00
4 00
2.50
x
3.00 Electronegativity,
x
80
c
70
t
0
60
1
:I
d.
0
70 60
d.
I 2.00
2.50
3.50
3.00
Electronegativity,
4.00
x
2 50
3 00 Electronegativity,
Figure 4. The effect of different basis sets and levels of theory on the correlation between deviations from planarity for H-N=O adducts W) STO-3G, r = and electronegativity of the adding radical. (a) (+ O.SOl;(b)(*-- -*)3-21G,r=0.759;(~)(0- -0)6-31G(d),r=0.773; the a-values for the following basis sets are plotted, but the least squares straight lineis not included,sinceit essentiallysuperimposeson the 6-31G(d) data: (+) 6-31G(d,p), r = 0.787; (A) 6-31+G(d), r = 0.795; (a), 6-31+G(d,p), r = 0.809; (d) (V-V) MP2/6-31G(d,p), r = 0.840.
-
2 00
-
x
Figure 5. The effect of different basis sets and levels of theory on the correlation bctwecn deivations from planarity for C H 3 - N 4 adducts and electronegativity of the adding radical. (a) (D- - W) STO-3G. r = 0.788;(b) (e--*)3-21G,r=0.775;(~) (0-- -0)6-31G(d),r=0.808; the a-values for the following basis sets are plotted, but the least-squares straight lineisnot includcd,sinceitessmtiallysuperimposeson the6-31G(d) line: (+) 6-31G(d,p), r = 0.807; (A) 6-31+G(d), r = 0.815; (o), 6-31+G(d,p), r = 0.821; (d) (V-V) MP2/6-31G(d,p), r = 0.799.
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1863
Nitroso Spin Adducts
TABLE 9
Formation Energies for Nitroso Spin Adducts 0 R' R*
+
+
I
H-N--R
H-N=O
CHJ-N=O
-
formation energy HNO spin adducts radical 'F 'OOH 'OH "=C
"4 'NH2 'CI *CF3 "*
H" 'SH 'CH2F 'CH=CH2 'CH3 CH2CH3 'PHI 'BH2 'SiH3 'Li 'H
0
I
CHa-N-R
formation energy CH3NO spin adducts
AESCP
Ah"
USCF
AhiP2
(kJ/mol) -64.7 +1.89 -74.6 -132.8
(kJ/mol) -155.7, -163.6 -35.3 -1 36.6, -146.9 -178.0, -137.7 +32.8 -163.0 -71.8, -93.2 -240.0 -351.7, -307.3 -370.4, -354.3 -90.2, -82.8 -234.8 -308.3 -222.5, -213.0 -224.1 -128.9 -385.5 -254.3 -227.9 -254.5, -245.2
(kJ/mol) -58.8 +19.6 -62.2 -118.1 +14.1 -106.0 -23.6
(kJ/mol) -160.8, -168.5 -3 1.3 -150.0 -175.0, -134.7 +33.7 -151.6 -7 1.2, -94.8
-272.0 -271.1 -53.7 -215.7 -245.2 -187.6 -185.8 -105.9 -368.7 -221.0 -196.9 -255.3
-340.2, -297.3 -357.1 -76.0 -222.3
-4.0 -127.0 -39.0 -250.6 -290.0 -293.3 -79.1 -238.7 -269.4 -209.9 -209.8 -136.9 -398.7 -250.1 -212.9 -277.0
-209.8, -201.0 -363.0 -237.1 -238.9, -229.7
a Where two values appear, the first is the AE obtained from single point MP2 calculations with the optimized geometry from the 6-31G(d) basis set, Le., MP2/6-3 1G(d,p)//HF/6-3 1G(d);the second is determined from energies of structures optimized at the MP2 level, i.e., MP2/631G(d,p)//MP2/6-3 lG(d,p).
of low electronegativity (
