Theoretical study of interstellar hydroxylamine chemistry: protonation and proton transfer mediated by H3+

Ž .Chemical Physics 244 1999 163–174www.elsevier.nlrlocaterchemphys

Theoretical study of interstellar hydroxylamine chemistry:protonation and proton transfer mediated by Hq

3

Pascal Boulet a,b, Francois Gilardoni a, Jacques Weber a,), Henry Chermette a,b,Yves Ellinger a,c

a Departement de Chimie Physique, UniÕersite de GeneÕe, 30 quai Ernest-Ansermet, CH-1211 GeneÕa 4, Switzerland´ ´ `b Laboratoire de Chimie Physique Theorique, Bat. 210, UniÕersite Claude Bernard LYON I et Institut de Recherche sur la Catalyse, UPR´ ˆ ´

5401 CNRS, 43, Bd du 11 NoÕembre 1918, F-69622 Villeurbanne Cedex, Francec Laboratoire d’Etude Theorique des Milieux Extremes, Ecole Normale Superieure, 24 rue Lhomond, F-75236 Paris Cedex 05, France´ ˆ ´

Received 14 December 1998

Abstract

Protonated species are known to play a key role for ion-molecule reactions in gas phase interstellar chemistry. AsŽ .hydroxylamine H NOH has never been observed as an interstellar molecule, a detailed theoretical investigation of the2

Ž Ž . .protonation of H NOH is carried out at high level of quantum chemical theories CCSD T and DFT-B3LYP . As2

protonation may occur directly by reaction with Hq or mediated by Hq, both processes are investigated on the nitrogen and3

the oxygen sites of hydroxylamine. The present results show that the N-protonated form is more stable than the O-protonatedone and that the protonation initiated by Hq is by far less exothermic than the other one. A particular attention is paid to the3

intramolecular rearrangement leading from H NOHq to H NOHq which involves a highly energetic transition state3 2 2

exhibiting proton bridged between N and O sites. As this barrier is too high to be easily overcome in the interstellar medium,an alternative process mediated by H and involving a bridged Hq as a transition state is considered. The calculations show2 3

that the corresponding activation energy is significantly lowered. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Interstellar hydroxylamine chemistry; Protonation; Proton transfer

1. Introduction

Hydrogen is by far the most abundant element inthe universe. Chemistry in space is therefore largelygoverned by hydrogen chemistry in all its forms,atomic, molecular or ionized according to the region

) Corresponding author. Tel.: q41-22-702-6530; Fax: q41-22-702-6518; E-mail: [email protected]

considered. In the diffuse medium where the radia-tion field is important, hydrogen is mostly ionizedand adds to neutral molecules as shown by theobservation of a number of protonated species such

q w x q w x q w x q w xas HCO 1 , N H 2 , HCS 3 , HOCO 3 ,2q w xHCNH 4–6 . Chemistry then proceeds via ion–

molecule reactions to build more complex organicions which are transformed in the end into neutralspecies, mostly through dissociative recombination

Ž q w x q w x.processes HCO 6 , HCNH 7 .

0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0301-0104 99 00151-2

( )P. Boulet et al.rChemical Physics 244 1999 163–174164

It is generally agreed that the proton involved inthese reactions is not a free proton but carried on byhydrogen clusters, the simplest of which is Hq. The3

energy released in the proton transfer can then bedissipated in the form of kinetic and vibrationalenergy of the products, namely H and the proto-2

nated species.Although not yet identified in the list of interstel-

lar NO containing molecules and seldom consideredw xin chemistry models 8 , hydroxylamine H NOH2

may be an important system, particularly because itpossesses two different sites with electron pairs whichmay lead to two stable protonated forms as in carbon

Ž q q.monoxide HCO and COH . Two different chainsof reactions could then be initiated whose possiblestarting points, H NOHq and H NOHq need to be3 2 2

investigated in detail.Experimental data on hydroxylamine are scarce

with the exception of vibrational spectra, where thebands are now rather well identified. The first suchstudy has been reported by Giguere and Liu in 1952`w x9 . Since then, numerous authors have proposed

w xassignments for the whole spectrum 10–12 . Itshould be said that the existence of the trans confor-mation of hydroxylamine has been assumed in the

w xinfrared study just mentioned 9 and has been con-w xfirmed in an X-ray analysis 13 of H NOH. How-2

ever, neither rotational barrier nor proton affinity areknown experimentally. Among the two protonatedforms, only the structural parameters of the N-proto-nated species have been reported by Shi and Wangw x14 based on an X-ray study of the crystal structureof hydroxylamonium chloride.

Since two decades hydroxylamine has been widelystudied at high levels of theory. In the early 1980s,

w xDel Bene et al. 15 performed MP4SDQ calculationson some protonated bases involving two heavy atoms,leading to the determination of proton affinities.

w xRastrelly and Cocchi 16 calculated the rotationalenergy profile of simple molecules such as hydroxyl-amine, correlating differences in stability and rota-tion barriers between similar molecules to thebond–bond, bond–lone pairs and lone pairs–lonepairs intramolecular interactions. Vibrational spectraand spectroscopic constants of small molecules have

w xbeen calculated at the MP2 level by Tyrell et al. 17 .These authors also carried out a detailed analysis ofthe frequency shifts due to isotopic changes. Bond

activation by protonation was studied by M. Alcamı́w xet al. 18 using a topological analysis of the molecu-

lar electronic density first introduced by Bader andw x w xEssen 19 , Bader et al. 20 and Wiberg and Bader´

w x21 . They showed that bond breaking may occur ornot, depending on the difference in electronegativitybetween the centre which undergoes protonation andits vicinal centre. Recently, Chung-Phillips and Jeb-

w xber 22 have reported a very accurate study at fullŽ . w x Ž .MP2 level, G2 MP2 theory 23 and full QCISD T

w x24 level of the rotational barrier of various speciesincluding hydroxylamine. They emphasized thechanges in calculated barriers due to basis sets andcorrelation treatment.

The aim of our work is to show how and where aproton can bind to hydroxylamine. Due to the factthat the structures of all protonated interstellar ionsknown so far are correctly predicted by examination

Ž .of the molecular electrostatic potential MEP of thew xparent neutral system 25 , this property together

w xwith the Fukui function 26 for the electrophilicattack have been used in a first step to determine thepreferred site. Then, a particular attention is paid tothe energetics of the attack of hydroxylamine by Hq

and Hq, which leads to the proton affinities on each3

site. For all these reactions, transition states, if any,are determined. In addition, we report the energybalance for electronic dissociative recombination re-actions which could neutralize and destroy the posi-tive ions formed.

2. Computational strategy

Determination of accurate geometries and ener-gies requires a convergent process in terms of bothbasis set extension and configuration space expan-sion. A systematic study of the equilibrium structureson the potential energy surfaces has been carried outusing theoretical approaches of increasing quality.The ab initio calculations were performed at the HFw x w x Ž . w x27–29 , MP2 30 , and CCSD T 24 levels oftheory whereas the DFT ones were carried out usingthe hybrid B3LYP functional. The basis set used was6-311qqGUU for all calculations. The saddle pointfor proton transfer between H NOHq and H NOHq

2 2 3

has been given a special attention and additional

( )P. Boulet et al.rChemical Physics 244 1999 163–174 165

w x w xcalculations at the CISD 31 and CASSCF 32,33 ,levels have been performed to ascertain the structureof the transition state and the activation barrier. ForCISD calculations, all the doubly occupied molecularorbitals were taken into account except for the twolowest occupied molecular orbitals which accommo-date the 1s electrons of oxygen and nitrogen. Thenall the valence electrons are used to build the excited

configurations. The active space of the CAS calcula-tion was composed, for H NOHq, of the NO bond-3

ing and antibonding orbitals, of the NH bonding andantibonding orbitals and the oxygen lone pair towhich the proton is transferred. For H NOHq, it was2 2

composed of the NO bonding and the antibondingorbitals, the OH bonding and the antibonding orbitalsand the nitrogen lone pair. Such an active space,

Ž .Fig. 1. Molecular electrostatic potential of hydroxylamine a.u. . The values of the MEP are plotted in the N–O–H plane of the molecule.From the inner to the outer the values are: isovolume 0.6 a.u., isocontours 0.55, 0.45, 0.35, 0.25 and 0.15 a.u.


which takes into account up to quadruple excitations,allows for a well-behaved description of simultane-ous bond breakingrbond formation mechanism ofthe proton transfer reaction. Geometry optimizationshave been performed within these schemes on

q q w xqH NOH , H NOH and H N PPP H PPP OH2 2 3 2

molecular cations. In view of the similarity of theŽ .results, only MP2, B3LYP and CCSD T methods

w q xhave been retained for probing the H PPP H NOH3 2

transition state.Ž .Zero-point energy ZPE corrections were ob-

tained at the HF, MP2 and B3LYP levels and scaledby 0.86, 0.93 and 0.95, respectively. These numberswhich correspond to the mean values of the ratiosbetween the calculated and experimental H NOH2

frequencies, are close to those derived recently fromw xa systematic study of typical organic molecules 34 .

Our best estimates of the energetics are obtainedŽ .taking the CCSD T values to which the high quality

B3LYP ZPE corrections are added.w xThe codes used in this study are Hondo8.0 35

w xfor CASSCF and CISD and Gaussian92r94 36 forall the other ones. The Fukui functions, at the localand condensed levels, have been evaluated using the

w xdeMon code 37,38 as modified at the University ofw xGeneva 39 .

3. Qualitative approach to protonation

Whereas the MEP is a well-known electrostaticw xproperty, the Fukui functions 26 correspond to the

response of the electron density to a perturbation onthe number of electrons.

Ž .The MEP of hydroxylamine Fig. 1 shows thatthe isosurfaces are more extended around the oxygenatom, with a smaller directional character than aroundnitrogen, due to the presence of two lone pairsinstead of one. However, the potential well is deeper

Ž y1in the nitrogen region y970.2 kJ mol againsty1 .y761.6 kJ mol for oxygen suggesting preferen-

q Ž .tial formation of H NOH Fig. 3-2a .3

Contrasting with the MEP, whose minimum indi-cates the most reactive site for the electrophilicattack, it is the maximum of the Fukui function thatcorrelates to this site. The corresponding condensed

w x Ž .Fukui function 39 Fig. 2 values amount to 0.403ey and 0.237 ey for the nitrogen and oxygen atoms,respectively.

This qualitative study shows that both approachespoint to a protonation occurring at nitrogen, whichwill be confirmed by the accurate calculations. Italso underlines that the local Fukui function is notonly a good index for predicting the position of an

Ž .Fig. 2. Fukui function for the electrophilic attack of hydroxylamine a.u. . The values of the function are plotted in the N–O–H plane of themolecule. The maxima are depicted using ' and l symbols and amount 0.085 a.u. and 0.159 a.u., respectively.


electrophilic attack but also shows its directionalityw x39 , here along the nitrogen lone pair.

4. The H NOHHHH system2

The qualitative picture presented above suggeststhat of the two protonated species which could beobtained through the reactions:

H NOHqHq™H NOHq 1Ž .2 3

H NOHqHq™H NOHq 2Ž .2 2 2

Ž .the product of reaction 1 should be favoured. Inorder to verify this result we calculated the proton

Ž .affinity of H NOH Fig. 3-1a at the nitrogen and2

oxygen sites. This property can easily be derivedfrom Table 1 where the energies of the differentsystems of interest to this study are reported. Asexpected, the proton affinity is far larger on the

Ž y1nitrogen atom ranging from 820.1 kJ mol accord-y1 .ing to HF to 802.0 kJ mol according to B3LYP .

Fig. 3. Molecular geometries. The labels of the hydrogen atoms correspond to those assigned in Tables 4 and 6 to the geometricalparameters.

()

P.B

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Table 1Ž UU a. Ž .Molecular energies 6-311qqG basis set pertinent to interstellar hydroxylamine chemistry a.u.

c cŽ . Ž .Molecules HF MP2 B3LYP CCSD T CCSD T rrB3LYP CASSCF CISDbŽ .NH OH trans 3-1a y130.989761 y131.426193 y131.726608 y131.417007 y131.4169462

bŽ .NH OH cis 3-1b y130.981964 y131.417786 y131.718491 y131.408844 y131.4087522bŽ .NH OH maximum-3-1c y130.977951 y131.414352 y131.715179 y131.405556 y131.4042052

q bŽ .NH OH 3-2a y131.302124 y131.733978 y132.032089 y131.727142 y131.727097 y131.382141 y131.4625753q bŽ .NH OH maximum-3-2b y131.300903 y131.732635 y132.030899 y131.725769 y131.340723 y131.4228693q bŽ .NH OH gauche 3-3a y131.264755 y131.693085 y131.993103 y131.687881 y131.6876372 2q bŽ .NH OH trans 3-3b y131.26149 y131.691605 y131.991501 y131.6869202 2

without ZPE y131.317922 y131.745034 y132.043634 y131.739065ZPE 0.056434 0.053435 0.052133 0.052133

q bŽ .NH OH local maximum-3-3c y131.26149 y131.691788 y131.991638 y131.6870882 2

without ZPE y131.317922 y131.744997 y132.043614 y131.739065ZPE 0.056434 0.0532 0.051971 0.051971

q bŽ .NH OH global maximum-3-3d y131.249084 y131.679962 y131.980698 y131.6752932 2q bw x Ž .NH ..H ..OH 3-4 y131.200531 y131.652679 y131.950943 y131.645065 y131.644821 y131.290436 y131.3713682q bw x Ž .NH ..H ..OH 3-5 y132.826973 y133.136444 y132.8122862 3

Ž .H C y1.122036 y1.160301 y1.169513 y1.158317 y1.1552042 `vŽ .H O C y76.03038 y76.293950 y76.437248 y76.265327 y76.2652442 2v

q Ž .H D y1.277443 y1.328865 y1.326231 y1.317234 y1.3172243 3hØ Ž .NH C y55.561718 y55.758100 y55.8815 y55.740528 y55.7405132 2vŽ .NH C y56.178238 y56.434680 y56.548458 y56.400475 y56.4004103 3v

Ø Ž .OH D y75.405602 y75.600209 y75.753967 y75.58802 y75.588013`h

aOtherwise mentioned all energies include ZPE corrections.bRefers to the label of Fig. 3.cCalculated with the 6-311GUU basis set. ZPE corrections not included.


The proton affinity on the oxygen atom only amountsy1 Ž .to 708.9 kJ mol averaging over all the values .

This confirms the trend previously evidenced fromthe index analysis. It is of interest to notice that the

Ž .proton affinities derived from the CCSD T and theŽ .CCSD T energies at B3LYP optimized geometries

Ž Ž . .hereafter referred to as CCSD T rrB3LYP areŽ y1 y1very similar 711.2 kJ mol and 710.7 kJ mol ,

. Žrespectively on O , indeed exactly equal 814.3 kJy1 .mol on N . This test will be further performed on

rotation and proton transfer barriers, both for neutraland protonated species.

4.1. H NOH2

The structures of hydroxylamine are presented inŽTable 2 for both its stable trans conformation Fig.

. Ž .3-1a and the cis local minimum Fig. 3-1b . Thegeometries obtained at the MP2 level are, as ex-pected, extremely close to those reported by Chung-

w xPhillips and Jebber 22 who used the 6-311qŽ .G 3df,2p basis set. More interesting is the compari-

Ž .son of B3LYP and CCSD T geometrical parame-ters. The agreement between the two methods isspectacular, not only comparing the bond lengths,but also concerning the bond angles.

Table 2Ž UU .Calculated geometry 6-311qqG basis set of the trans and

˚Ž .cis conformation of H NOH Bond length in A, angle in deg2

aŽ .HF MP2 B3LYP CCSD T ExpbNH OH trans 3-1a2

NH 1.001 1.017 1.018 1.020 1.016NO 1.395 1.434 1.444 1.444 1.453OH 0.940 0.960 0.963 0.961 0.962HNH 107.1 105.9 106.2 105.5 107.1HNO 105.7 104.3 104.2 104.0 103.2NOH 104.9 101.8 102.8 101.8 101.4HNOH 123.3 124.5 124.4 124.8

bNH OH cis 3-1b2

NH 1.001 1.017 1.019 1.021NO 1.388 1.423 1.431 1.433OH 0.943 0.964 0.967 0.965HNH 109.0 107.9 108.4 107.2HNO 108.7 107.9 108.0 107.4NOH 109.5 107.5 108.4 107.4HNOH 59.3 58.2 58.5 57.5

a w xTaken from Ref. 8 .bRefers to the labels of Fig. 3.

The potential energy curve for internal rotation ofhydroxylamine has been widely described over the

w xpast 16,17 . The most stable conformation ofŽ .H NOH is the trans one Fig. 3-1a , which is about2

y1 Ž .21.3 kJ mol lower than the cis Fig. 3-1b accord-Žing to B3LYP calculations see Table 1 for the

.energies with ZPE correction included . These twoŽ .structures have been optimized at the CCSD T lev-

els and the energy difference amounts to 21.4 kJmoly1. Similarly, the calculation procedure previ-

Ž .ously denoted as CCSD T rrB3LYP gives an en-ergy difference of 21.5 kJ moly1.

Ž .The transition state for the rotation Fig. 3-1c lies8.7 kJ moly1 and 8.6 kJ moly1 above the cisconformer according to respectively B3LYP and

Ž . Ž .CCSD T whereas CCSD T rrB3LYP gives 11.9kJ moly1 which is slightly overestimated. However,

Ž .B3LYP and CCSD T rrB3LYP energetics stillcompare well with each other and this behaviour willbe further accredited by the proton transfer fromH NOHq to H NOHq as it will be demonstrated in2 2 3

a forthcoming section.

4.2. H NOH q3

Two geometries corresponding respectively to theminimum and maximum of the potential energy sur-face, for the rotation around the NO bond, have been

Ž .found see Fig. 4a . They are both of Cs symmetry.ŽThe stable conformer is of staggered type Fig.

.3-2a . The geometrical parameters are gatheredin Table 3. According to our best estimatesŽ .CCSDTrrB3LYP , this conformer is about 3.1 kJmoly1 more stable than the transition state which

Ž .exhibits an eclipsed conformation Fig. 3-2b . Thefact that the most stable conformation is the stag-gered one can be rationalized by the minimization ofthe steric repulsion between hydrogens. As there isno direct lone pair repulsions occurring during therotation of H NOHq, the barrier is lower than that3

of H NOH rotation.2

The energetical path from the rotational transitionstate to the stable conformer requires to modify thetorsion angle by 628. This is slightly more than onecould expect by considering an approximate sp3

hybridization of nitrogen orbitals, leading to an angleof 608. This is related to the non-equivalence of thethree hydrogen atoms bonded to the nitrogen. There-


Ž .Fig. 4. a Potential energy curve for internal rotation of N-proto-nated hydroxylamine. The labels refer to the structures depicted in

Ž .Fig. 3. b Potential energy curve for internal rotation of O-proto-nated hydroxylamine. The labels refer to the structures depicted inFig. 3.

fore, they feel different steric interactions with thehydrogen bonded to the oxygen, and with the oxygenlone pairs. The maximum discrepancy between the

˚three NH bond lengths amounts only to 0.002 A.This implies a small distortion of the molecule whichis more perceptible in bond angles as less energy is

Table 3Ž UU . qCalculated geometry 6-311qqG basis set of H NOH3

a ˚Ž . Ž .3-2a Bond length in A, angle in deg

Ž .HF MP2 B3LYP CCSD T

NH 1.013 1.027 1.029 1.0291

NH 1.014 1.029 1.031 1.0312

NO 1.367 1.397 1.406 1.407OH 0.951 0.971 0.974 0.972H NO 105.7 104.9 104.7 104.81

H NO 112.2 112.3 112.4 112.22

H NH 108.2 108.3 108.3 108.41 2

H NH 110.0 110.4 110.3 110.42 2

NOH 108.9 106.6 107.4 106.3H NOH 62.3 62.6 62.6 62.52

aRefers to the labels of Fig. 3.

involved in an angular deformation than in a stretch-ing one; the NH umbrella is tilted with respect to3

the NO axis. Finally, the symmetry of the cation isno longer C but C . The same situation is found3v s

w xfor the isoelectronic H COH 16 .3

The identification of each vibrational frequency iseasy as there is no important combination of the

Ž .normal modes Table 4 . As there are no experimen-tal data available on this species, the scale factorsderived from H NOH calculations have been applied2

to the protonated systems.Ž y1 .The NH bending modes 1550 cm and the2

Ž y1 .two NH stretching modes 3200 cm are quasidegenerate. As explained above, the three hydrogenatoms are not equivalent. Then, these modes, whichbelong to the E symmetry of the C point group of3v

an ideal NH system, split into the AX and the AY3

irreducible representations of the effective C points

group. The two NH bending modes exhibit a very2Ž y1small splitting about 1 cm , except for MP2 which

Table 4q Ž y1 .Scaled vibrational frequencies of H NOH cm and their3

Ž y1 .intensities km mol

Mode symmetry Method and scaling factor

HF 0.86 MP2 0.93 B3LYP 0.95YTorsion A 285.2 305.3 316.5

179.0 162.1 161.2XNO stretch A 1030.3 989.4 962.9

20.4 19.1 15.2XNH wagg A 1088.9 1089.6 1093.62

52.4 48.7 52.3YNH rock A 1142.1 1133.9 1130.52

20.3 20.7 23.0XNOH bend A 1388.9 1387.0 1400.8

47.1 53.4 51.4XUmbrella A 1512.8 1500.6 1508.8

56.8 47.9 47.8YNH bend A 1542.5 1523.5 1553.52

44.1 41.6 45.9XNH bend A 1543.2 1532.5 1554.52

49.2 35.9 40.5XNH s stretch A 3090.6 3139.2 3148.9

41.9 33.7 30.1YNH a stretch A 3163.0 3226.8 3217.6

145.4 138.9 124.9XNH a stretch A 3167.7 3240.4 3242.8

176.2 168.6 158.1XOH stretch A 3493.4 3500.6 3504.2

206.6 152.1 153.7


y1 .gives 9 cm as compared with the two NH stretch-Ž y1 .ing modes about 15 cm , except for HF .

Comparing the vibrational frequencies of the neu-tral and protonated molecule, we see that the NOHbending mode exhibits a ‘blue-shift’ of 80 cmy1 andthe NH2 bending is slightly ‘red-shifted’ by 30cmy1. The NO stretching mode is shifted towardhigher wave numbers. This can be explained by theNO bond length shortening, as a consequence ofprotonation. All the NH stretching modes are shiftedtoward low wave numbers, due to a lengthening ofthese bonds. This has been rationalized by Alcamı et´

w xal. 18 in their topological analysis. As protonationoccurs on the nitrogen atom, there is a transfer of theelectron density in the NO bonding region corre-sponding to a depletion in the NH and OH bonds.

4.3. NH OH q2 2

The potential energy surface of H NOHq ex-2 2

hibits one or two minima, depending on the level ofŽ .theory see Fig. 4b . The global minimum found in

Ž .all calculations Fig. 3-3a has a hydrazine-type con-Ž .formation see Table 5 . The trans conformation

Ž .with C symmetry Fig. 3-3b is either a transitions

state or a secondary minimum erroneously consid-ered as the absolute energy minimum in the pastw x15 . There are also two energy paths from onehydrazine like conformation to the other. Each onehas a mirror plane of symmetry. The one with a high

Table 5Ž UU . qCalculated geometry 6-311qqG basis set of H NOH2 2

a ˚Ž . Ž .3-3a Bond length in A, angle in deg

Ž .HF MP2 B3LYP CCSD T

NH 1.004 1.020 1.023 1.0241

NH 1.005 1.021 1.023 1.0252

NO 1.441 1.461 1.482 1.486OH 0.957 0.977 0.978 0.9773

OH 0.958 0.979 0.981 0.9784

H NO 103.3 102.3 102.0 101.21

H NO 106.1 106.0 105.5 104.92

H NH 111.5 109.9 110.1 109.01 2

NOH 111.4 108.3 108.9 107.63

NOH 119.8 117.3 118.6 116.74

H NOH 137.9 141.8 140.0 141.61 3

H NOH 32.5 24.5 26.1 20.42 4

aRefers to the labels of Fig. 3.

energy barrier, corresponding to an eclipsed C tran-sŽ . y1sition state Fig. 3-3d , is 32.4 kJ mol above theŽ Ž . .minimum CCSD T rrB3LYP . The other, corre-

Žsponding to the C star-shaped conformation Fig.s.3-3b may or may not go through a local minimum

Ž .Fig. 3-3c .The energy path from the global minimum to the

Ž .local one if any involves a transition state with C1Ž .symmetry Fig. 3-3c . The difference between this

latter structure and the local minimum is very smallŽin terms of torsion angle for example 308 in MP2

.calculations . In terms of rotational energy, Fig. 4bshows that the potential energy surface is very flat inthe vicinity of the local minimum. Including the ZPEcorrection, the two conformers have roughly the

Ž .same energy see Table 1 , the difference amountingonly to 10y4 a.u. This points out that the localminimum, if any, cannot be detected in gas phaseexperiments.

Some vibrational modes of this molecule are moremixed than for H NOHq, especially the NH and3 2

OH bending and the NH and OH stretching2 2 2Ž .modes see Table 6 . This mixing might explain why

their frequencies are so close on to the other.Comparing with the N-protonated hydroxylamine,

the NO stretching mode is significantly ‘red-shifted’,contrary to that of H NOHq. This can be rational-3

ized by the lengthening of the NO bond as evidencedw xin Ref. 18 . As the protonation occurs, the electron

density required for the bond formation is recoveredfrom the bonding region of the molecule, namely theNO bond.

4.4. H NOH PPP H q2

According to the large abundance of Hq in inter-stellar clouds, a direct protonation of the hydroxyl-amine may be inferred. The energetics of the reac-

Ž . Ž .tions 1 and 2 mentioned above correspond toproton affinities on nitrogen and oxygen, respec-tively. Attempts to locate transition states for theproton attacks have aborted. As generally admittedfor ion–molecule processes this exothermic reactionoccurs without transition barrier. By contrast, a tran-sition state has been found for the intramolecularproton transfer from oxygen to nitrogen, pointing out

Ž .to a bridged hydrogen structure Fig. 3-4 . As ex-pected, the calculations show a significant increase


Table 6q Ž y1 .Scaled vibrational frequencies of H NOH cm and their2 2

Ž y1 .intensities km mol

Mode and intensity Method and scaling factor

HF 0.86 MP2 0.93 B3LYP 0.95

Torsion 361.8 339.9 345.935.9 47.4 36.2

NO stretch 582.4 699.9 620.5367.1 251 276.4

NH rock 748.5 831.2 759.92

45.1 67.1 67OH rock 1004.3 1037.3 1015.72

95.8 73.6 77.2NOH bend 1135.3 1168.3 1158.5

71.2 62.1 64.5NH wagg 1278.8 1286.3 1280.92

11.9 15.7 12.1OH bendqNH bend 1541.6 1537.6 1554.42 2

75.1 97.5 93.3OH bendqNH bend 1577.8 1568.6 1582.12 2

71.4 27.4 36.6NH s stretch 3185.5 3239.7 3240.12

42.6 35.4 38.3NH a stretchqOH 3287.1 3352.4 3347.42 2

s stretch 92.1 51.3 62.4OH s stretchqNH 3370.3 3368.6 3393.82 2

a stretch 271.1 240.7 206.2OH a stretch 3455.5 3463.1 3489.42

469.8 398.2 387.7

˚Žof the NO bond length 1.575 A according toŽ ..CCSD T so as to accommodate the bridging hydro-

˚ ˚Ž .gen OHs1.168 A; NHs1.262 A .The activation barrier required to transform

H NOHq into H NOHq is 106.1 kJ moly1 at2 2 3Ž .the MP2 level whereas B3LYP, CCSD T and

Ž .CCSD T rrB3LYP give very close values amount-ing to 110.7 kJ moly1, 112.4 kJ moly1 and 112.4 kJmoly1, respectively. According to our best estimates,the oxygen to nitrogen internal proton transfer is an

Ž y1 .exothermic process D Hsy100 kJ mol whichrequires at least 110 kJ moly1 to overcome thetransition state barrier. It is hardly conceivable thatsuch intramolecular rearrangement could occur in theinterstellar conditions.

From a computational point of view, it is impor-Ž .tant to report that CCSD T calculations at the opti-

mized B3LYP geometries lead to the same transitionŽ .energy as pure CCSD T . The general trend emerg-

ing from this series of tests is that, whatever the

Ž .molecular species considered cationic or neutral , allenergy differences involving proton affinity, rotation

Ž .and transition barriers at the CCSD T level of the-ory are reproduced with a remarkable accuracy by

Ž .the CCSD T rrB3LYP procedure. Whenever possi-ble, this strategy will be used in the forthcomingstudies.

5. The H NOHHHH system2 3

It is well known that Hq does not exist in aque-ous solution as a free species but is attached to waterclusters. Similarly, it is considered that in the regiondominated by molecular hydrogen, Hq is actuallyattached to hydrogen clusters.

Ž . Ž .The protonation reactions 1 and 2 that consid-ered Hq as a free proton are then simplistic modelsof the actual reactions implying Hq as proton car-3

rier:

H NOHqHq ™H NOHqqH 3Ž .2 3 3 2

H NOHqHq ™H HOHq qH 4Ž .2 3 2 2 2

Ž .The energetics Table 1 of these reactions showŽ y1that they are also exothermic y397.0 kJ mol and

y1 Ž Ž ..y293.9 kJ mol , respectively CCSD T , thoughŽ y1 . Ž .far less ;420 kJ mol than for the reactions 1

Ž . Ž .and 2 . Then, reaction 3 is thermodynamically lessŽ . Ž .favoured than reaction 1 . However, in reaction 1

the excess energy which cannot be transferred to athird body could dissociate the molecule, whereas in

Ž .reaction 3 it can be evacuated into the kineticenergy of the products. On the other hand, reactionŽ .1 generates three more vibrational frequency modeswhich could in part help dissipating the exothermic

Ž .energy of the reaction, whereas the reaction 3 andŽ .4 generate one new mode only.

To rationalize the protonation process occurring atw q x wthe nitrogen site, the H PPP NH OH and H3 2 2

qxPPP NH OH systems have been optimized with3

Hq and H in the vicinity of H NOH and H NOHq,3 2 2 3

respectively. The sampling of the potential surfacehas been performed along the direction suggested bythe Fukui function for the preferential electrophilicattack. Both systems converge towards the sameminimum corresponding to a long range interactionbetween H NOHq and H . This shows that the3 2


protonation on this site is a dynamical process whichdoes not require an activation energy. Similarly, forprotonation occurring at the oxygen site, the optimi-

w qx w qsations of the H NOH PPP H and H NOH2 3 2 2xPPP H systems lead to long range interaction be-2

q Žtween H NOH and H . In both cases N and O2 2 2.sites , attempts to localize transition states for the

proton transfer or long range interacting structuresbetween H NOH and Hq have aborted.2 3

Due to a high energy barrier, we have seen thatthe proton transfer cannot occur as such. However,we can assume that this process could take placewith H as a mediator. This leads to2

H NOHq qH ™H NOHqqH 5Ž .2 2 2 3 2

where, in a first step, H extracts Hq from the2w q xoxygen atom, forming a H N PPP H PPP OH tran-2 3

sition state, which in a second step undergoes Hq

transfer to the nitrogen atom. This transition stateŽ .has been identified Fig. 3-5 . It lies 80.2 kJ

y1 Žmol higher than the reactants according to theŽ . .CCSD T rrB3LYP calculation . The path has been

Žchecked through an IRC mapping Intrinsic Reaction.Coordinates performed at the B3LYP level that

connects the protonated systems. The barrier is sig-nificantly lowered compared with the direct processŽ y1 .112.4 kJ mol . However, it is still a high barrierto overcome and even in the presence of H this2

rearrangement could probably not occur. In otherwords, assuming that H NOHq and H NOHq are3 2 2

interstellar molecules, both forms may coexist inthese extreme media.

The abundance of these species will be governedby dissociative recombination reactions with sur-rounding electrons which are known to be the mostefficient processes for the destruction of positive

Žions. In this context the energetics of reactions Ta-.ble 1 :

H NOHqqey™NH qOH Ø3 3

D Hsy686.2 kJ moly1

H NOHq qey™NH Ø qH O2 2 2 2

D Hsy834.8 kJ moly1

show that the recombinations are highly exothermicand lead to very stable interstellar molecules andradicals.

No matter how H NOH is synthetized, the pre-2

sent study suggest that the sequence of protonationqdissociative recombination reactions could resultin very low abundance of these protonated ions.

6. Concluding remarks

The protonation of hydroxylamine leads to twostable species, H NOHq and H NOHq, the N-pro-3 2 2

tonated one being roughly 100 kJ moly1 more stablethan the O-protonated one. Whatever the site consid-ered, the direct attack of Hq is highly exothermicŽ y1 .e.g., y800 kJ mol on N and the excess energywould most probably dissociate the molecule, con-trary to the Hq mediated protonation which is about3

400 kJ moly1 less exothermic.The rearrangement of H NOHq to H NOHq has2 2 3

been studied. It is evidenced that neither the intra-molecular rearrangement nor the bimolecular reac-tion initiated by a H molecule can occur even2

though the barrier is significantly lowered by about30 kJ moly1 in the latter case. The transition stateslie at about 110 kJ moly1 and 80 kJ moly1 above thereactants. Consequently, the two protonated formsmay coexist in the interstellar medium, provided theyare not immediately destroyed by dissociative re-combinations. Rotational constants have been ob-

Ž .tained from CCSD T calculations, 139.0, 24.5 and25.4 GHz for H NOHq and 144.7, 22.4 and 22.13

GHz for H NOHq, which should, however, encour-2 2

age laboratory investigations to identify those ions.

Acknowledgements

This work has been carried out in the context of aconvention for a joint PhD Thesis between the Uni-versity Claude Bernard Lyon I and the University ofGeneva. This work is part of the Project 20-49037-96of the Swiss National Science Foundation. The sup-port of CNRS-IDRIS is acknowledged.

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Theoretical study of interstellar hydroxylamine chemistry: protonation and proton transfer mediated by H3+