Comparison of density functional calculations of CNO2, NNO2 and CNF2 dissociation energies

THEO CHEM Journal of MolecularStructure(Theochem)388 (1996) 51-55

ELSEVIER

Comparison of density functional calculations of C-NO2, N-NO2 and C-NF2 dissociation energies P e t e r P o l i t z e r * , Pat L a n e Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA

Received 1 December1995;accepted7 February1996

Abstract

We compare the effectiveness of three exchange/correlation functional combinations (Becke/Lee, Yang and Parr; Becke-3/ Lee, Yang and Parr; Becke-3/Perdew-Wang 91) for computing C-NO2, N-NO2 and C-NF2 bond lengths and dissociation energies. The Becke-3/Perdew-Wang 91 is found to give the best results, although for C-NF2 bonds the differences are less important than for C-NO2 and N-NO2. The presence of NO2 and NF2 on the same carbon considerably weakens the bond to each. Keywords: Density functional calculations; Dissociation energy

1. Introduction

The stabilities and impact/shock sensitivities of energetic materials have frequently been related to the strengths of C-NO2 and/or N-NO2 bonds [113]. For designing and evaluating new compounds, it is accordingly important to be able to determine computationally the dissociation energies of these bonds. This is complicated, however, by the wellknown difficulties involved in describing the electronic structures of nitro derivatives [14-19]. The necessity of including a high level of electronic correlation coupled with the relatively large size of many energetic molecules makes non-local density functional theory an attractive computational option [20,21]. We have accordingly investigated the effectiveness of three different exchange/correlation functional combinations in reproducing the C-NO2 and * Correspondingauthor.

N-NO2 dissociation energies in H3C-NO2 and H2N-NO2. In view of the interest in the difluoramino group, NF2, as a substituent in energetic molecules, particularly propellants [22-27], we have included the C-NF2 bond in H3C-NF 2 ill this study. Finally, in response to a report that gem-nitro/difluoramino compounds have been synthesized [28], we have examined the effects of the interaction of these groups upon the C-NO2 and C-NF2 dissociation energies in H2C(NO2)NF2.

2. Methods

All calculations were carried out with the density functional option of GAUSSIAr~94 [29]. The exchange functionals were the Becke (B) [30] and the Becke three-parameter hybrid (B3) [31]; the correlation functionals were the Lee, Yang and Parr (LYP) [32] and the Perdew-Wang (PW 91) [33]. The three

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P. Politzer, P. Lane~Journal of Molecular Structure (Theochem) 388 (1996) 51-55

combinations of these that were tested are designated as B/LYP, B3/LYP and B3/PW91. The basis set was the 6-31G °'.

3. Results The optimized geometries obtained by these three procedures for the four undissociated molecules are given in Table 1; also included are experimentally determined structures for H3C-NO2 [34]

and H2N-NO2 [35]. The calculated total and zeropoint energies of the molecules and their fragments are in Table 2, and the resulting dissociation energies for the five bonds of interest are in Table 3. Available for comparison with the latter are experimental data indicating that the H a C - N O 2 dissociation energy is about 59 kcal mo1-1 [18,36-38] and a calculated value of 53.6 kcal mo1-1 for H 2 N NOz [39], obtained by the ab initio G2 procedure [40], which was designed to reproduce energetic properties for first- and second-row molecules to

Table 1 Optimized geometries Molecule

Distance/.~ or angle/deg

Computationalprocedure B/LYP

HaC-NO2

B3/LYP

B3/PW91

C-N N-O C-H N-C-H C-N-O H-C-H O-N-O

1.521 1.246 1.095 106.8-108.0 116.9.117.0 110.4-112.9 126.1

1.500 1.227 1.088-1.091 106.9-108.1 117.0 110.4-112.8 125.9

1.492 1.221, 1.222 1.087-1.090 107.3-108.6 116.4, 117.6 109.5-112.0 126.0

H3C-NF2

C-N N-F C-H N-C-H C-N-F H-C-H F-N-F

1.484 1.450 1.099 106.8-110.8 102.7 110.0-111.1 102.1

1.466 1.414 1.091-1.093 107.0-110.7 103.5 109.9-111.0 102.0

1.461 1.404 1.091-1.093 107.1-110.9 103.7 109.7-111.0 102.1

H2N-NO2

N-N N-H N-O N-N-H N-N-O H-N-H O-N-O

1.430 1.024 1.242 108.3 115.9 114.4 128.0

1.394 1.014 1.225 109.7 116.2 116.2 127.5

1.384 1.012 1.220 110.1, 110.2 116.2 116.7 127.5

H2C(NF2)-NO2

C-NO2 C-NF2 N-O N-F C-N-F C-N-O F2N-C-H O2N-C-H O-N-O F-N-F H-C-H

1.535 1.488 1.243-1.239 1.435-1.454 99.9-102.8 113.8-118.7 107.7-112.1 106.9-110.5 127.4 102.5 111.9

1.509 1.502 1.469 1.463 1.220-1.225 1.215-1.220 1.401-1.414 1.391-1.404 101.0-103.8 101.2-104.0 113.9-118.8 113.9-118.8 108.0-111.9 108.0-112.1 107.1-107.6 107.0-107.6 127.3 127.3 102.4 102.6 111.5 111.4

a Experimentalgeometries are from Ref. [34] for H3C-NO2 and Ref. [35] for H2N-NO2.

Experimentala 1.489 1.224 107.2

125.3

1.381 1.007 1.232 109.7 113.6 120.9 132.7

P. Politzer, P. Lane~Journal of Molecular Structure (Theochem) 388 (1996) 51-55

53

Table 2 Calculated total and zero-point energies Molecule

H3C-NO 2 H 3C-NF 2 H2N-NO2 H2C(NF2)-NO2

Total energy/hartrees

Zero-point energy/kcal mol-1

B/LYP

B3/LYP

B3/PW91

-244.97210 -294.16286 -261.00795

-245.01337 -294.20987 -261.03782

-244.92207 -294.10201 -260.94239

-498.64241

-498.68993

293.50855 -244.30984 -205.06886 -254.25109 -39.80978 -55.85596

-293.54460 -244.34232 -205.07221 -254.26248 -39.84288 -55.87898

B/LYP

B3/LYP

B3/PW91

30.23 28.39 23.61

31.33 29.56 24.77

31.50 29.70 25.02

-498.50622

29.60

31.35

31.62

-293.43276 -244.25058 -204.99491 -254.17009 -39.82752 -55.85708

19.81 21.35 5.14 3.48 18.25 11.47

20.69 22.09 5.54 3.81 18.68 11.91

20.75 22.29 5.66 3.90 18.71 12.00

H2C-NF 2 H2C-NO2 "NO2 •NF 2 "CH3 'NH2

w i t h i n a n a v e r a g e a b s o l u t e d e v i a t i o n o f less t h a n 1.6 kcal mo1-1.

4. Discussion F o r p r e s e n t p u r p o s e s , the key structural f e a t u r e

s e e n in T a b l e 1 is the s i g n i f i c a n t d e c r e a s e ( a n d i m p r o v e m e n t ) in the C - N O 2 a n d N - N O 2 b o n d l e n g t h s in g o i n g f r o m B / L Y P to B 3 / L Y P to B 3 / P W 9 1 o p t i m i z a t i o n ; the g r e a t e s t c h a n g e o c c u r s b e t w e e n B / L Y P a n d B 3 / L Y P . In contrast, the v a r i a tion in the C - N F 2 d i s t a n c e s , w h i l e in the s a m e direction, is m u c h s m a l l e r .

Table 3 Calculated dissociation energies a Dissociation process

Method

AE/kcal mo1-1

H3C-NO2 ~ H3C" + "NO2

B/LYP I33/LYP B3/PW91 Experimental b

51.8 54.6 55.4 58.5-59.5

H3C-NF 2 ---* H3C. + "NF2

B/LYP B3/LYP B3/PW91

57.3 58.5 58.4

H2N-NO2 -'* H2N' + 'NO2

B/LYP B3/LYP B3/PW91 G2 c

45.2 47.0 49.4 53.6

H2C(NF2)-NO2 ' ~ H2(~-NF2 + 'NO2

B/LYP B3/LYP B3/PW91

36.1 40.8 44.1

H2C(NF2)-NO2 "* H2(~-NO2 + "NF2

B/LYP B3/LYP B3/PW91

46.4 48.0 48.3

"Zero-point energies are taken into account. b Refs. [18,36-38]. c Ref. [391.

P. Politzer, P. Lane~Journal of Molecular Structure (Theochem) 388 (1996) 51-55

54

The computed dissociation energies show corresponding trends. For H3C-NO2, the B/LYP result is too low by about 7 kcal mol-1; however, it increases in increments of 2.8 and 0.8 kcal mol-1 upon proceeding to the B3/LYP and B3/PW91 functionals. Similarly, the HzN-NO2 dissociation energy is about 8.4 kcal mol -a too low at the B/LYP level, but increases by 1.8 and 2.4 kcal mo1-1 in going to B3/LYP and B3/PW91. On the other hand, the total variation in the H3C-NF2 dissociation energies is only 1.2 kcal mo1-1. The results obtained for H2C(NO2)NF2 follow the same qualitative patterns as observed for H3C-NOz and H3C-NF2: the C-NO2 dissociation energy increases significantly from B/LYP to B3/LYP to B3/PW91, the C-NF2 only slightly. What is very striking, however, is that both the C-NOz and the C-NF2 dissociation energies are considerably less in H2C(NO2)NF2 than in H3C-NO2 and H3C-NF2; at the B3/PW91 level, the differences are 11.3 and 10.1 kcal mo1-1, respectively. This mutual bond-weakening effect may be attributable in part to the destabilizing interaction between NO2 and NF2 groups substituted on the same carbon [41]; we estimate its magnitude to be approximately 4.4 kcal mo1-1 (B3/PW91). From the standpoint of energetic materials, this bondweakening should introduce a note of caution, because of the evidence linking sensitivity and instability to ease of C-NO2 and/or N-NO2 bond cleavage [1-13]. Indeed the gem-nitro/difluoramino compounds that have been prepared were reported to be highly sensitive [28].

5. Summary Our results indicate that: 1. Both C-NO2 and N-NO2 dissociation energies can be improved by 1-4 kcal mol -t by going from B/LYP to B3/LYP, and by a similar increment upon proceeding to B3/PW91. These improvements appear to be related to having more accurate optimized bond lengths. 2. C-NF2 distances and dissociation energies are less affected by these changes in functionals, but do vary in the same directions as the C-NO2 and N-NO2 results.

3. The simultaneous presence of NO2 and NF2 on the same carbon considerably weakens the bond to each.

Acknowledgements We greatly appreciate the assistance of Ms. Monica C. Concha and Dr. Jane S. Murray, and the financial support of the Office of Naval Research, through contract No. N00014-95-1-0028 and Program Officer Dr. Richard S. Miller.

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