Conformational potential energy surface of BrOONO

Antonija Lesar a,*, Sa�ssa Prebil a, Max M€uuhlh€aauser b, Milan Hodo�ss�ccek a,c

a Department of Physical and Organic Chemistry, Institute Jo�zzef Stefan, Jamova 39, Ljubljana SI-1000, Sloveniab Institut f€uur Physikalische und Theoretische Chemie der Universit€aat Bonn, Wegelerstrasse 12, Bonn 53115, Germany

c Center for Molecular Modeling, National Institute of Chemistry, Hajdrihova 19, Ljubljana SI-1000, Slovenia

Received 14 August 2002; in final form 5 November 2002

Abstract

Cis-perp and trans-perp conformations with respect to N–O and O–O bonds were found to be the only stable ones

on the BrOONO potential energy surface using the CCSD(T)//B3LYP method with the 6-311G* basis set. The energy

for the cis-perp form is 2:0 kcal mol�1 lower than for the trans-perp form while the saddle point connecting the twominima is 9:0 kcal mol�1 above the cis-perp level. A comparison of the relative energetics for stationary points on theBrOONO, ClOONO, and HOONO conformational potential energy surfaces is discussed.

� 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

Already in 1975 Wofsy et al. [1] pointed out that

bromine compounds participate in the destruc-

tion of stratospheric ozone. The first report on

stratospheric significance of bromine nitrate,

BrONO2, was made by Spencer and Rowland [2]

on the basis of UV and IR absorption character-istics measurements and the estimation of the rate

of formation from BrO and NO2. The kinetics

of reaction was studied at room temperature by

discharge-flow-mass spectrometric and flash-pho-

tolysis-UV absorption techniques for low and high

pressure conditions, respectively [3]. The reaction

was found to be pressure dependent and later ki-

netics studies established the negative temperature

dependence of the low-pressure [4] and the high-

pressure [5] rate constant of reaction. While none

of the works [3,4] have been addressed to analyze

the isomer formation, it has nevertheless been

speculated that it will not occur. Gas-phase

structural studies for BrONO2 by the combined

use of electron diffraction intensities and rotational

constants has been provided by Casper et al. [6]. Ina thermal decomposition study of bromine nitrate

by IR spectroscopy, Orlando et al. [7] have con-

sidered the possibility of product formation

other than BrONO2 in a recombination reaction

but they found no obvious evidence of isomer

formation.

The role of BrONO2 as a reservoir for active

bromine in the lower stratosphere is now wellrecognized. The major loss processes of BrONO2in the stratosphere include photolysis at UV

wavelengths [8] during the day and heterogeneous

reactions during the night [9]. The quantum yield

Chemical Physics Letters 368 (2003) 399–407

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* Corresponding author. Fax: +386-1-251-93-85.

E-mail address: antonija.lesar@ijs.si (A. Lesar).

0009-2614/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0009-2614 (02 )01888-2

measurements of NO3 production in the UV

photolysis of BrONO2 have shown that besides

NO2, NO3 is also an important photolytic product

[10]. Although numerous laboratory studies have

been carried out to elucidate the equivalent reac-

tion of ClO, i.e., ClOþNO2, there still remainsconflicting indirect evidence of the presence of

more than one isomer of chlorine nitrate. On the

other hand, longstanding controversy over the

existence of peroxynitrous acid, HOONO, an iso-

mer of nitric acid, HONO2, was just recently re-

solved with the first direct spectroscopic

observation of HOONO [11] produced in reaction

of NO2 with OH radicals.Thus, we are not aware of any experimental

conclusions that different isomers of BrONO2 may

be formed. Likewise, we do not know of any the-

oretical investigations that have been attributed to

isomeric species of BrONO2, while there exist a

few theoretical studies devoted to the structural,

vibrational and thermochemical parameters of

BrONO2 [12,13]. In this study, we present the first,as far as we know, ab inito calculations on the

BrOONO isomer and its conformational forms by

determining their equilibrium or first-order saddle

point structures, vibrational frequencies, and rel-

ative energies. It is also of interest to compare and

contrast the results with those of ClOONO and

HOONO. The ab initio methods used in the

present investigation are described in Section 2.Section 3 contains the results and discussion, while

the conclusions are summarized in Section 4.

2. Computational methods

Ab initio molecular orbital calculations were

performed using the GAUSSIANAUSSIAN 98 program [14].The geometrical parameters of the BrOONO

conformational forms were obtained using the

second-order M€ooller–Plesset (MP2) method [15],and the DFT approach was obtained using the

Becke three-parameter nonlocal exchange func-

tional [16] with the nonlocal correlation of Lee,

Yang, and Parr (B3LYP) [17,18]. For minima

structures, the single and double excitation cou-pled-cluster method including a perturbation esti-

mate of the effects of connected triple excitations

(CCSD(T)) [19–21] was also used. The standard

triple f plus polarization 6-311G(d) basis set, asimplemented in the computer program [14], was

employed in the optimizations.

The harmonic frequencies were computed from

analytical derivatives at the MP2 and B3LYPlevels for all species, employing the geometries

calculated at these levels of theory, in order to

characterize the nature of the stationary points on

the conformational potential energy surface and to

determine the zero-point vibrational energies

(ZPE).

The anharmonic frequencies were calculated at

the B3LYP level using the 6-311G(d) basis set andthe VSCF [22] method as implemented in the

GAMESS [23] package.

The intrinsic reaction coordinate (IRC), which

follows the minimum-energy path from the first-

order saddle point in both directions, has been

calculated to confirm the connections between the

maxima and the stable forms of BrOONO.

In order to obtain the energy differences withhigh accuracy, we performed single-point calcula-

tions for stationary points on conformational po-

tential energy surface at the CCSD(T) level with

the 6-311G(d) basis set using optimized geometri-

cal parameters obtained with the B3LYP/

6-311G(d) level of theory.

3. Results and discussion

3.1. Geometries and vibrational frequencies

On the conformational potential energy surface

of bromine peroxynitrite, BrOONO, we have

characterized seven stationary points. The geo-

metrical arrangement of nuclei, corresponding tothese points, are shown in Fig. 1. Their structural

parameters, obtained without any symmetry

constraint in the optimizations, are provided in

Table 1. For convenience, the same notation is

used as McGrath and Rowland [24] introduced

for the conformers of HOONO and ClOONO.

They characterized the conformers with respect to

both the sðONO0OÞ and s0ðNO0OBrÞ dihedralangles. The nonplanar cis-perp conformer

(s; s0 ¼� 0; � 90) was found to have the lowest

400 A. Lesar et al. / Chemical Physics Letters 368 (2003) 399–407

energy. The other minimum corresponds to

the trans-perp conformer (s; s0 ¼� 175;� 90). Thelatter conformer results from the rotation of theO0OBr group around the N–O bond. The maxi-

mum energy structures state accompanying this

rotation has the perp–perp conformation (s; s0 ¼� 88; � 85). The examination of the rotation ofthe OBr group around the O0–O bond reveals

four additional stationary points verified to be

maximum energy structures with one imaginary

vibrational frequency. When the OBr unit of thecis-perp conformer is rotated, the corresponding

maxima have cis–cis (s; s0 ¼ 0; 0) and cis–trans

(s; s0 ¼ 0; 180) structures while maxima related tothe trans-perp conformer have trans–cis (s; s0 ¼180; 0) and trans–trans (s; s0 ¼ 180; 180) arrange-ments of the ONO0O and NO0OBr torsional

angles.

From Table 1 it can be seen that for the morestable cis-perp conformational forms of BrOONO,

the MP2 and DFT (B3LYP) methods give sub-

stantially different values for both the bond lengths

and the valence bond angles. The deviation of the

NO0 and OBr bond lengths between the two

methods exceeds 0.4 and 0.2 �AA and is about 20� forthe ONO0 and NO0O bond angles. The geometry

prediction of the more reliable and extensive

CCSD(T) calculations agrees better with that of

the DFT (B3LYP) level of theory. Thus, we must

conclude that the geometry of the cis-perp form is

not adequately described by the MP2 method.

CCSD(T) calculations are also employed for thetrans-perp conformation and all three methods

yield comparable results for the geometrical pre-

diction. These calculations suggest that for our

systems under investigation, containing one third-

row atom, the DFT results are to be preferred over

those resulting from the MP2 approach. The in-

adequacy of MP2 calculations for the cis-perp

conformer may arise from the overemphasizedcontributions from electron correlation or, more

likely, from singlet–triplet instability of the HF

wavefuction as a reference. Likewise, an unusually

long N–O bond, and ONO and NOO bond angles

around 90� at the MP2 level were found in acomputational study on alkali peroxynitrite [27].

The geometrical parameters for the maximum

energy conformeric forms calculated at the MP2and B3LYP levels are quite close to each other.

The bond lengths differ by less than 0.03 �AA, exceptthat for the perp–perp transition state, where the

NO0 and OBr bond distances differ by 0.06 and 0.4�AA in the two methods, respectively. Also, the dif-ferences in bond angles are small, on the average

less than 2�, with one exception: in the cis–trans

conformer the MP2 and DFT values for the NO0Oangle differ by 4�.The N–O bond length in the NO2 molecule is

1.1944 �AA and the bond angle is predicted to be

134.2� at the B3LYP/6-311G(d) level of treatment,which is in excellent agreement with experimental

values [28] of 1.1934 �AA and 134.1�, respectively.When a comparison is made between NO2 and the

lowest energy structure of BrOO0NO we can see ashrinkage of the N–O bond to 1.133 �AA and an

elongation of the N–O0 bond to 1.684 �AA; the bondangle is distinctly compressed to 110�. Also, theO–Br bond (1.9861 �AA) is elongated compared toits value in the OBr radical, calculated as 1.764 �AAat same level of theory. The NO0O and O0OBr

bond angles are predicted to be 108� and 113�,respectively.The bond lengths for the trans-perp conformer

are changed by less than 0.08 �AA compared to the

Fig. 1. Conformers of bromine peroxynitrite as calculated by

the B3LYP method.

A. Lesar et al. / Chemical Physics Letters 368 (2003) 399–407 401

Table 1

Optimized geometries (in �AA and (�)) of the equilibrium structures and transition statesa

Species Method rðONÞ rðNO0Þ rðO0OÞ rðOBrÞ \ðONO0Þ \ðNO0OÞ \ðO0OBrÞ s s0

Cis-perpb MP2 1.181 2.146 1.216 2.208 89.1 90.6 117.8 0.1 90.0

B3LYP 1.133 1.684 1.330 1.986 109.8 107.6 113.2 )3.6 91.3

CCSD(T) 1.150 1.609 1.366 1.964 111.5 108.2 111.2 )3.9 90.3

Trans-perpb MP2 1.151 1.597 1.381 1.922 109.0 05.6 110.5 175.4 89.4

B3LYP 1.142 1.598 1.359 1.950 108.6 107.1 112.1 173.5 93.2

CCSD(T) 1.159 1.540 1.389 1.939 108.5 105.7 110.3 175.4 90.7

Cis–cis MP2 1.144 1.597 1.428 1.928 119.3 129.2 118.5 0 0

B3LYP 1.141 1.566 1.400 1.943 117.9 129.4 120.5 0 0

Cis–trans MP2 1.175 1.430 1.485 1.859 115.6 109.0 103.5 0 180

B3LYP 1.165 1.420 1.476 1.865 117.0 112.2 105.9 0 180

Perp–perp MP2 1.141 1.864 1.297 2.073 109.8 101.2 114.1 88.2 85.5

B3LYP 1.125 1.800 1.313 2.033 108.8 105.2 114.0 84.5 88.6

Trans–cis MP2 1.156 1.537 1.427 1.905 108.8 114.8 115.9 180 0

B3LYP 1.148 1.532 1.395 1.929 109.8 115.2 116.7 180 0

Trans–trans MP2 1.166 1.492 1.470 1.858 108.6 102.0 104.1 180 180

B3LYP 1.154 1.508 1.448 1.868 108.3 104.6 106.3 180 180

aObtained with the 6-311G(d) basis set.bRotational constants (MHz) and dipole moments (D) determined using CCSD(T)/6-311G(d) geometries are for cis-perp: Ae ¼ 8450, Be ¼ 1729, Ce ¼ 1547, l ¼ 1:71

and for trans-perp Ae ¼ 12373, Be ¼ 1331, Ce ¼ 1248, l ¼ 1:53.

402

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Table 2

Unscaled harmonic vibrational frequencies (cm�1) of bromine peroxynitrite conformersa

Species Method Description of normal modesb

ONstr OO0str ONO0

bend OBrstr NO0str O0OBrbend NO0Obend ONO0Otor NO0OBrtor

Cis-perp MP2 1846 1458 676 560 443 307 265 210 125

B3LYP 1930 976 778 559 412 312 231 192 89

Anharm 1884 950 760 550 404 304 265 197 89

Intc 406 82 19 19 74 1 42 58 0.2

Trans-perp MP2 1824 939 715 562 378 318 228 171 86

B3LYP 1892 995 729 547 414 333 237 170 77

Anharm 1856 977 717 539 405 324 231 180 75

Int 472 40 198 37 99 4 66 0.4 0.3

Cis–cis MP2 1860 811 738 580 317 272 202 209i 155

B3LYP 1870 823 771 596 289 288 221 200i 152

Cis–trans MP2 1664 937 719 613 486 378 229 189 56i

B3LYP 1755 947 738 603 439 398 207 170 64i

Perp–perp MP2 1952 985 604 428 385 245 119 154i 74

B3LYP 1983 1005 621 462 415 281 180 163i 84

Trans–cis MP2 1779 898 765 570 326 287 209 216i 157

B3LYP 1851 914 790 559 359 329 213 170i 149

Trans–trans MP2 1727 929 744 608 466 259 208 162 92i

B3LYP 1829 967 731 623 498 239 205 162 110i

aObtained with the 6-311G(d) basis set.bAssignment based on inspection of normal mode animation.c IR intensity (km mol�1).

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368(2003)399–407

403

cis-perp conformer. The main difference is related

to the ONO0O torsional angle being 174� for thetrans-perp conformer. From inspection of the

geometrical parameters of the maximum energy

conformeric forms listed in Table 1, one can see

that major structural changes occur only with re-spect to the two dihedral angles.

To facilitate the experimental identification of

BrOONO, rotational constants and dipole

moments for both minimum energy structures at

the CCSD(T)/6-311G(d) level of calculations are

included in the footnotes of Table 1.

The calculated harmonic frequencies of the two

stable BrOONO conformers are provided in Ta-ble 2 along with those of the corresponding

maximum energy conformeric forms. These fre-

quencies are needed to verify the true nature of

stationary points and to provide the ZPE. The

vibrational frequencies are calculated at the MP2

and B3LYP levels of theory with the 6-311G(d)

basis set. The agreement between the calculated

frequencies at the two levels is satisfactory, exceptfor that of the cis-perp conformer what can be

expected due to the difference in the MP2 geom-

etry. Thus, the MP2 frequencies of the cis-perp

structure are not reliable. While the cis-perp and

trans-perp frequencies in the B3LYP calculations

are very similar, any vibration can be used to

distinguish between them. But the remarkable

difference is related to the computed IR intensitiesof the ONO0 bending vibration and the ONO0O

torsion being substantially more intense for

the trans-perp and cis-perp conformers, respec-

tively. An inspection of normal mode suggests

that the reaction vector corresponding to the

imaginary frequency for the perp–perp transition

can be identified as the ONO0O torsional vibra-

tion rather than the NO0OBr torsion. This can beunderstood from the much heavier bromine mass

than that of oxygen. As expected, the eigenvec-

tors corresponding to the negative eigenvalue of

the force constant matrix for the cis–cis and cis–

trans maximum energy forms are similar, as are

those of the trans–cis and trans–trans maximum

energy, respectively. The former is mainly com-

posed of the ONO0O torsion and twisting while inthe latter eigenvector the twisting mode is less

pronounced.

3.2. Relative energetics of BrOONO conformers

The total electronic energies, zero-point ener-

gies (ZPE), and relative energetics with regard to

the cis-perp conformer at the MP2 and B3LYPlevels of treatment, including the ZPE for minima

and maxima, are summarized in Table 3. Also,

results of the more elaborate CCSD(T) calcula-

tions are presented in the table to obtain the most

reliable energy ordering of the structures deter-

mined. For maxima, only CCSD(T) single-point

calculations at the MP2 and B3LYP geometries

were performed. We have seen above that theB3LYP structural parameters for the cis-perp

conformer are more consistent with CCSD(T)

values than MP2 values and thus it is not sur-

prising that the CCSD(T)//MP2 and CCSD(T)//

B3LYP energies differ by 5:4 kcal mol�1. For allother species this difference does not exceed

0:3 kcal mol�1. The CCSD(T)//MP2 energies arenot quoted in the table. Further, the difference inenergy between CCSD(T) and CCSD(T)//B3LYP

is only 0:5 kcal mol�1 for both the cis-perp and thetrans-perp conformers. We believe that the ener-

gies of maximum energy conformeric forms are

reasonably well predicted by the CCSD(T)//

B3LYP level of treatment.

As shown in Table 3, the MP2 and B3LYP

energies indicate and the CCSD(T) energy con-firms that the cis-perp is actually the lowest energy

conformer. It lies 2.3 and 2:0 kcal mol�1 at

B3LYP and CCSD(T)//B3LYP levels, respectively,

lower than the trans-perp conformer. The MP2

value of 7:0 kcal mol�1 is significantly higher. It isworth mentioning that the relative energies for

maximum energy conformeric forms obtained at

the B3LYP and CCSD(T)//B3LYP levels are infair agreement with each other.

In the following text we consider the results ob-

tained at the CCSD(T) level for maximum energy

conformeric forms. The energy barrier for rotation

around the N–O0 bond is 9:0 kcal mol�1. The OBrrotation around the peroxide bond in the cis-perp

conformer leads to barriers of 15:7 kcal mol�1 (cis–cis) and 5:6 kcal mol�1 (cis–trans). The corre-sponding barriers for rotation in the trans-perp

conformer were found to be 11:1 kcal mol�1

(trans–cis) and 7:9 kcal mol�1 (trans–trans).

404 A. Lesar et al. / Chemical Physics Letters 368 (2003) 399–407

3.3. Comparison of relative energies of the XOONO

(X¼Br, Cl, H)

It is of interest to compare the relative ener-

getics for BrOONO with those of the chlorine andhydrogen analogues. The conformational prefer-

ence of ClOONO and HOONO was investigated

by McGrath and Rowland [24] at the MP2 level of

theory. The minimum energy conformer of HO-

ONO has also been studied by Tsai et al. [25]

employing various methods, and by Sumathy and

Peyerimhoff [26] who studied various HOONO

structures and their possible decomposition prod-ucts employing DFT(B3LYP) and CASSCF

calculations. For a reliable comparison we have

done the recomputations here at the B3LYP/

6-311G(d,p) level. Namely, we have seen in the

previous section that the B3LYP/6-311G(d) rela-

tive energies for the various BrOONO confor-

mational forms agree fairly well with the

corresponding CCSD(T)//B3LYP energies. The

comparison in Fig. 2 clearly shows that the con-

formational potential energy surface is qualita-

tively identical for ClOONO and BrOONO. Forboth isomers, the most stable form is cis-perp,

which is only 2:3 kcal mol�1 lower than the trans-perp conformer. The energy barrier for the inter-

conversion between them is 8.9 and 9:2 kcal mol�1

for chlorine and bromine analogues, respectively.

The relative energies of other corresponding

maxima on the conformational potential energy

surface of the bromine and chlorine compoundsare similar as well but the difference is less than

1 kcal mol�1.

In contrast to ClOONO and BrOONO, the

minimum energy structure of HOONO is found to

Table 3

Absolute energies, E (hartree), zero-point energies, ZPE (kcal mol�1), and corrected relative energies, DE0 (kcal mol�1), of bromine

peroxynitrite conformersa

Species Method E ZPE DE0

Cis-perp MP2 )2852.10745 8.4 0.0

B3LYP )2854.41175 7.8 0.0

CCSD(T) )2852.14719 0.0b

CCSD(T)//B3LYP )2852.14647 0.0b

Trans-perp MP2 )2852.09483 7.5 7.0

B3LYP )2854.40791 7.7 2.3

CCSD(T) )2852.14398 2.0

CCSD(T)//B3LYP )2852.14312 2.0

Cis–cis MP2 )2852.07357 7.1 20.0

B3LYP )2854.38919 7.2 13.5

CCSD(T)//B3LYP )2852.12050 15.7

Cis–trans MP2 )2852.08897 7.5 10.7

B3LYP )2854.40155 7.5 6.1

CCSD(T)//B3LYP )2852.13709 5.6

Perp–perp MP2 )2852.08407 6.9 13.2

B3LYP )2854.39606 7.2 9.2

CCSD(T)//B3LYP )2852.13125 9.0

Trans–cis MP2 )2852.07969 7.1 16.1

B3LYP )2854.39558 7.4 9.7

CCSD(T)//B3LYP )2852.12814 11.1

Trans–trans MP2 )2852.08457 7.3 13.3

B3LYP )2854.39684 7.5 9.1

CCSD(T)//B3LYP )2852.13337 7.9

aObtained with the 6-311G(d) basis set.b For CCSD(T) and CCSD(T)//B3LYP relative energies the ZPEs obtained at the B3LYP level of treatment were used.

A. Lesar et al. / Chemical Physics Letters 368 (2003) 399–407 405

be the cis–cis conformer, with its cis-perp being

only 0:8 kcal mol�1 higher, followed by the trans-perp with 3:4 kcal mol�1 relative energy. The low-energy cis–cis structure in HOONO is presumablydue to an intramolecular hydrogen bond which is,

of course, not present in the halogen analogues.

The process of interconversion of the cis–cis to the

cis-perp conformer involves the cis-gauche struc-

ture as the maxima. At the B3LYP level this

structure is isoenergetic with the cis-perp con-

former. The energy barrier that corresponds to

rotation around the N–O0 bond amounts to15:1 kcal mol�1, which is significantly higher

compared to the halogen analogues. The rota-

tional barrier calculated in this work is in reason-

able accord with the value of 12:5 kcal mol�1

reported by Sumathy and Peyerimhoff [26]. On the

other hand, all the relative energies of the maxi-

mum energy structures for rotation around the

peroxide bond are significantly lower than thecorresponding barriers in ClOONO and BrO-

ONO.

Let us finally consider the relative stability of

the most stable XOONO isomeric form with the

corresponding XONO2. The BrONO2 is calcu-

lated to be 24.8 and 22:5 kcal mol�1 more stablethan BrOONO at the B3LYP/6-311G(d) and

CCSD(T)/6-311G(d) levels of theory, respectively.

These energy differences are nearly equal to those

found for the analogous chlorine compounds, be-

ing 24.1 and 22:3 kcal mol�1, respectively. At theB3LYP/6-311G(d,p) level of calculations, the

HONO2 is stabilized by 30:0 kcal mol�1, which is

in good agreement with previously reported val-ues: 30:9 kcal mol�1 at the B3LYP/6-311++G(d,p)level [26] and 29:5 kcal mol�1 at the CCSD(T)/CBS level [29].

4. Summary

The conformational potential energy surface ofthe BrOONO isomer of bromine nitrate was in-

vestigated by the MP2, B3LYP and CCSD(T) ab

initio electronic structure methods. The main re-

sults can be summarized as follows.

Two stable conformers of BrOONO were found

differing in their OO0NO torsional angle. The

lower-energy one is a nonplanar cis-perp con-

former, i.e., the Br atom is around 90� out of theplane formed by the other atoms. The other con-

former, trans-perp, having only 2:0 kcal mol�1

higher energy, results from approximately 180�rotation of the O0OBr group around the N–O0

bond. The maximum energy structure accompa-

nying this interconversion has a perp–perp con-

formation and the calculated energy barrier is

9:0 kcal mol�1. Rotation of the O–Br bond in thecis-perp conformer around the O0–O peroxide

bond leads to barriers of 15:7 kcal mol�1 (cis–cis)and 5:6 kcal mol�1 (cis–trans). The correspondingbarriers for rotation of the trans-perp conformer

were found to be 11:1 kcal mol�1 (trans–cis) and7:9 kcal mol�1 (trans–trans).Comparison of structures and relative energies

at the B3LYP/6-311G(d,p) level of treatmentclearly shows that the conformational potential

energy surface for ClOONO is qualitatively

identical to that of BrOONO. In both systems,

the most stable form is cis-perp which is calcu-

lated to be only 2:3 kcal mol�1 lower than thetrans-perp conformer in ClOONO. The energy

barrier for the interconversion between the two

stable structures and the relative energies of othercorresponding maxima on the conformational

potential energy surfaces are similar in BrOONO

Fig. 2. Relative energetics of bromine peroxynitrite (BrOONO)

conformers and comparison with chlorine peroxynitrite (ClO-

ONO) and peroxynitrous acid (HOONO), B3LYP/6-311G(d,p)

level of calculations (c, g, t and p are abbreviation for cis,

gauche, trans and perp, respectively).

406 A. Lesar et al. / Chemical Physics Letters 368 (2003) 399–407

and ClOONO as well, the difference being less

than 1 kcal mol�1. On the other hand, the mini-

mum energy structure of HOONO is the cis–cis

conformer, next is the cis-perp conformer being

only 0:8 kcal mol�1 higher, followed by the trans-perp conformer with 3:4 kcal mol�1 higher rela-tive energy. The lowest structure might be favored

by some intramolecular hydrogen bonding. The

process of interconversion of the cis–cis to the cis-

perp conformer involves the cis-gauche structure

proven to be the first-order saddle point. At the

B3LYP level of treatment this structure is isoen-

ergetic with the cis-perp conformer. The relative

energy of maximum energy structures for rotationaround the N–O0 bond is higher while those for

rotation around the O0–O bond are significantly

lower than the corresponding energies of ClO-

ONO and BrOONO.

Acknowledgements

This work was funded by the Ministry of Ed-

ucation, Science and Sport of Slovenia, Grant No.

P-544 and partly by the NATO collaborative

linkage Grant EST.CLG.977083. The authors

thank Prof. S.D. Peyerimhoff for valuable discus-

sion and critical reading of the manuscript.

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