HOC+: An observable interstellar species? A comparison with the isomeric and isoelectronic HCO+, HCN and HNC

Chemical Physics 60 (1981) l-10 North-Holland Publishing Company

HOC+: AN QBSEIWABLE llNTERSTELLAR SPECIES? A CQMPARIS8N WITH THE ISO~~C AND ISOELECFRONIC MCN AND MlrdC

IWO”,

Ross H. NOBES and Leo RADOM Research School of Chemistry. Australian National Uxicersity, Canberra, 2600, Australia

Received 15 April 1981

Ab initio molecukr orbital calcdations with large basis sets and with incorporation of electron correlation are used to examine the HOC’/HCO’ potential energy surface. The estimated equilibrium stmcture of HOC? has r.(C-O) = l.l%r: 0.003 A andr&l-H) = 0.988 rtO.003 A leading to a J = la 0 rotational transition frequency of 89.0+ 0.8 GHz. HOC+ is found to tie 157 kJ mol-’ above HCO’ with a barrier to rearrangement of 190 ‘kJ mol-‘. Comparative data for the HNC/HCN system are presented.

1. Introdwtion Interstellar radio emission from hydrogen cyanide (HCN) was first detected [l] just over a decade ago. Shortly afterwards, it was proposed [Z, 3] that unidentified lines at 90.66 GHz and 89.19 GHx were due to hydrogen isocyanide (HNC) and the formyl ion (HCO*) respectively. Support for these assignments came from accurate quantum chemical calculations of the structure of each species [4-5, 6-71 and final confirmation from Iaboratory measurements of their microwave spectra @-lo, 111. HCO’ has assumed additional importance since being proposed [12] as a key species in interstellar molecule formation via gas-phase ion-molecule reactions. The present study seeks to characterize HOC*, the remaining member of this isoelectronic series, and to investigate its possible existence as a metastable species in the interstellar medium. To this end we report high quality calculations of (i) the equilibrium geometry of HOC+, leading to a prediction of the J = 13 0 rotational transition frequency; 03Ol-0104/81/0000-0000/$02.50

0 North-Holland

(ii) the reaction profile for the rearrangement of HOC’ to the more stable formyl ion; and (iii) the energetics of a possible mode of formation of HOC+. To provide comparative data and to assist in assigning error bars to our results for HOC&, calculations have also been performed for the HCN, HNC and HCO’ systems including au examination of the I-DIG + HCN rearrangement. Of particular relevance is the fact that experimental equilibrium structures of HCN, HNC and HCO+ are available 113, 14, 151. By carrying out parallel calculations of the geometries of these species with known structures, advantage can be taken of the systematic nature of the errors in ab initio geometry predictions to yield improved predictions of the structure of HOC*. Such a procedure has been successfully utilized on previous occasions 17, 15-U]. Much of the previous ab initio work on HOC+ has been concerned with the energetics and mechanism of protonation of carbon monoxide [19-231. Several authors [19,23-251 present structures and relative energies of HCO’ and HOC” obtained at the Hartree-Fock f$fF) level. A post-HF calculation of the

2

RX. Nobes, L. Radom I’HOC’:

geometry of HOC has been presented [26]; the reported structure is compared with our results below. To the best of our knowledge, no detailed study of the potential energy surface relating to the rearrangement of HOC’ to HCO+ has hitherto been published.

2. Method Calculations were carried out using a modified version [27] of the GAUSSIAN 80 system of programs [28]. Electron correlation effects have been included via second (MP2) and third (MP3) order F&her-Plesset perturbation theory trea?ments [29] in which the core molecular orbitals have been frozen [30]. For the HI? calculations, stationary points were obtained using analytical gradient procedures. For the post-HF calculations, minima and saddle points were located using a parabolic interpolation procedure and numerical gradient program [31] respectively. Several basis sets of increasing size have been used in the present study: these are the split-valence 4-31G set [32] and three @-polarization sets termed 6-31G** [33], 6SllG** [34] and 6-31lG”**. The last basis set, introduced for the first time here, consists of the s- and p-functions of the 6-311G** set together with two (rather than one) sets of dfunctions centred on each heavy atom (exponents 1.084 and 0.361 on C, 1.581 and 0.527 on N, 2.238 and 0.746 on 0); the contraction scheme can be designated (llsSp2d/_Sslp) + [4s3p2d/3slp]. In order to correct relative energies for effects of zero-point vibrations, harmonic frequencies have been determined using HF calculations with the 4-31G basis set. Previous calculations [34] of the structures of some one- and two-heavy atom hydrides have shown that geomerric& parameters obtained at the MP3 level using the 6-311G** basis set are ’ quite close to experimental values. In addition, a study [3S] of the equilibrium structure of the water molecule using a large gaussian basis set has shown that the calculated geometry at the MP3 level is comparable in quality to that

An obseruabie intersiellnrspecies?

obtained from an all single- and double-excitation configuration interaction (SD-CI) approach. Finally, we note that in previous studies of the HNC-, HCN rearrangement [36, 371 and similar 1,2-hydrogen-shift reactions [36], it has been shown that MP3 values of isomerization and activation energies are quite close to those Jbtained from more complete correlation treatments. Throughout this paper, total energies are given in hartrees and bond lengths in Angstroms. As an example of the notation used, the description MP3 /6-31G”*//RHF/4-31G refers to a third-order Moller-Plesset calculation using the 6-3lG** basis set on a structure: which has been optimized at the restricted Hartree-Fock level using the 4-3 1G basis set.

3. Results and discussion 3.1. Equilibrium structure of HOC’ In order to obtain a reliable value of the J =

1 + 0 rotational transition frequency of HOC’, an accurate estimate of the equilibrium structure of this ion is required. Presented in table 1 are calculated geometries and total energies of HOC+ and the isoelectronic systems HCN, HNC and HCO’, obtained at both HF and MP3 levels using a sequence of basis sets of increasing size. Included in table 1 are the most reliable theoretical and experimental structures previously reported. The experimental equilibrium geometry of HCO’ [15] is somewhat less certain than that of HCN [13] or HNC [14] since the anharmonic force fieId of HCO’ is not well known. AIso included in table 1 are the errors in the caIculated geometrical parameters for the three systems with known experimental structures. Trends in these errors for the various theoretical levels are evident: (i) As expected [39], the HI? bond lengths are too short by =l-2%. (ii) At the MP3 level, the 6-31G** basis set overestimates the length of the bond between the heavy atoms (by =0.4%) while the larger

3 Table 1 Equilibrium geometries and total energies of HXY systems Species

Level

Energy

r&X--HI

r,wY) value

error”

value

error ”

HCN

RHF/4-31G ” RHF/6-31G** MP3/6-31G** MP3:6-311G** MP3/6-311G*** ref. [5] d, ref. [7] =I ref. [38] n expt. *

1.140 1.133 1.158 1.151=) I.146 1.150 1.148 1.151 1.153

-1.1 -1.8 eo.4 -0.2 -0.6

1.051 1.059 1.064 1.067 ‘) 1.066 1.066 1.064 F.065 1.065

-1.3 -0.6 -0.1 i0.2 +0.1

-92.73193 -92.87114 -93.16614 -93.19864 -93.21900 -93.19241 -93.21701 -93.19290

HNC

RHF/4-31G” RHF/6-31G** MP3/6-31G** MP3/6-311G** Mp3/6-3llG’** ref. [5] d1 ref. [38]’ expt. h,

1.163 I.155 1.175 1.168 1.163 1.170 1.168 1.169

-0.5 -1.2 +a.5 -0.1 -0.5

0.979 0.984 a.994 0.996 0.996 0.996 0.995 0.994

-1.5

-92.71678 -92.85961 -93.14080 -93.17524 -93.19566 -93.16916 -93.16893

I-X0+

RHF/4-3 IG ” RI-IF/&31G** MP3/6-31G** hfP3/6-3 I lG** MIP3/6-311G*** ref. [6] d’ ref. [7] =) e.Xpt. i,

1.098 1.087 1.110 1.099 1.097 1.1045 1.101 l.lOS-1.106

-0.7 -1.7 tQ.4 -0.6 -0.8

1.078 1.086 1.090 1.093 1.093 LO95 LOS0 1.056-1.098

-112.77931 -112.96809 -113.25726 -113.30803 -113.32955 -113.3064 -113.33025

HOC’

RHF/4-3 IG b’ RHF/6-31G** MP3/6-31G** MP3/6-311G3* hfP3/6-3llG*** ref. [26] ‘)

1.160 1.142 1.161 1.152 1.148 1.159

0.976 0.975 0.989 0.988 O.Y90 0.976

-112.74464 -112.92143 -113.19937 -113.24564 -113.26737 -113.0135

-1.0 0.0

+0.2 10.2

*) Percentage deviation from experimental value. ‘) RHF/4-31G structures from refs. [23-251. cl Ref. [34]. d, SD-C1 calculations with an (lls6pld/6slp)-r[66s3pld/3slp] basis set. =’ SD-C1 calculations with a (13sSp2d/Ss2p)-, [7sSp2d/4s2p] basis set. ” Coupled-cluster calculations with a (9sSpld/5slp)+[5s3pld/3slp] basis set. FJ Ref. 1133. “‘Ref. II4]_ ” Ref. [15]. ‘) SD-C1 calculations with a basis composed of (1ls6pld/6slp)~[5s3pId/3slp] plus two off-centre functions.

6-31 lG** and 6-3 1 lG**” sets underestimate thii length and slightly overestimate the X-Ii Iength. (iii) In general, the errors become more systematic with higher levels of theory, i.e. on going from EIF to MP3 and with increasing basis set size.

By applying corrections for the systematic errors at the various theoretical levels, we can obtain estimated structures at each of r/x levels examined (table 2). Structures obtained with the various basis sets at the MP3 level show pIeasing agreement; yheu averaged these yield a best estimate of the equilibrium structure of HOC’

4

Table 2 Applied correction of HOC+ Level

R.H. Nobes, L. Radom

factors and resultant estimated structures

r,(O-HI

rLC-0) correction

value

factor =’ N-F/4-31G RHF/6-31G** MF’3/6-31G** iMP3/6-311G** MT3/6-311G***

/ HOC+:

iO.8 i-l.6 -0.4 d-O.3 eO.6

correction

vahe

factor b, 1.169 1.160 1.156 1.155 1.~155

il.4 CO.8 +O.l -0.2 -0.2

0.990 0.983 0.990 0.986 0.988

a) Percentage,

based on average r&X-Y) error for HCN, HNC and HCO’ (see table 1). ” Percentage, based on average r.fX-H) error for HCN and HNC (see table 1).

An obsemble

interstehr

shown in table 3. In order to estimate the J = 1 + 0 rotational transition frequency of H’%“C+, the rotation-vibration correction must be takerrinto account. Estimates of this correction for HCN and HNC are 200 MHz and 155 MHz, respectively [40, 141. Kraemer and

Diercksen [7] calculate this quantity to be 189 MHz for HCN and 246 MHz for HCO’; they then empiricahy correct these values to obtain 258 MHz and 321 MHz, respectively. Assuming that the rotation-vibration constants of HOC’ wih be similar in magnitude to those of the other three species, we estimate the correction as 250 f 100 MHz. This yields a B. for HOC’ of 44.5 f 0.4 GHz and a rotationa! transition frequency of 89.0 f 0-g GHz. 3.2. R&rangement

with r&C-G) = 1.155 A and r,(O -H) = 0.988 A. The estimated uncertainty in each length is 0.003 A. Comparison of this structure with th& of Herbst et al. [26] shows some unexpected discrepancies. The r,(O-H) length of ref. [26] is some 0.012 a shorter than our value, and the r&Z-O) length 0.004 A longer. When we perform an IZHF/6-3llG** calculation at the optimum I-IF geometry of ref. [26], we obtain an energy which is 16 millihartrees lower than that of the previous study. This indicates that the basis set chosen in ref. [26] may not be fully appropriate to the present problem. Herbst et al. [26] have also excluded the 3u molecular orbital (which is primarily C-O bonding) from their correlation treatment; Firhen we repeat our MP3/6-311G** calculations with this orbital frozen we find a significant reduction in r,(C-G) (1.047 & and a smaller reduction in r,(O-H) (0.986 A). Finally, we note that even excluding the 3~ orbital from our treatment, we obtain a correlation energy (235 mihihartrees) significantly greater than that (83 milhhartrees) of ref. [26]. In the light of these arguments, we regard our estimated HOC’structure as the most reliable prediction currently avaiIable. Using our estimated structure, the equilibrium rotational constant B, of each isotopic species of HOC+ has been calculated; the results are

species?

of HOC+

to the fomyl

ion

Of crucial importance in deciding whether HOC+ will be an observable interstellar species is the magnitude of the barrier to intramolecular rearrangement to the more stable formyl ion. This 1,2-hydrogen-shift reaction, together with the HNC+HCN rearrangement, has been studied at a number of theoretical levels. Geometries and total energies of the transition structures, obtained at the RHF/4-31G, XHF/6-31G** and MP3/6-31G** levels, are given in table 4. The geometry of the HNC+ HCN transition structure has been determined previottsly at the HF level [36, 41, 42j and more elaborately by Pearson et al. [S] using an SD-C1 treatment with an (lls6pld/6sfp)+ Table 3 Equilibrium Species H’bO’ZC+ H’*o’2C+ H’601’C* H’SO’3C+ D’60’*C+ D’SO’2C+ D’%=C+ D’ao:3Cf *) ~ncetinty

rotational

constants of HOC’ Rotational

constant ‘) (GHz)

44.8 43.3 42.9 41.5 38.0 37.2 36.5 35.6 is h-O.3 GHz in each case.

R.H. Nobes, L. Radom / HOC’: Table 4 Geometries HXC+HCX

and total energies of transition structures for rearrangements

Level

r. (C-X)

r, (C-H)

r= (X-Ii)

Energy

TS+lNC+HCN RHF/4-31G” RKF/B-‘JIG** MP3/6-:lG** ref. [5]

1.183 1.169 1.187 1.181

1.210 1.152 1.163 1.171

1.394 1.469 1.430 1.430

-92.62502 -92.19662 -93.08436 -93.1135

TS:HOC++HCORHF/4-31G RHF/6-31G** MP3/6-31G**

1.141 1.122 1.150

1.350 1.266 1.281

1.456 1.436 1.379

-112.62201 -112.83485 -113.13122

a) Ref. [36].

[6s3pld/3slp] basis set. The structure obtained in this latter study is included in table 4; agreement with our MP3/6-31G** result is satisfactory. Note that the lengths of the partial X-H and C-H bonds are quite sensitive to the level of theory used. Vibrational frequencies and zero-point energies of the stable isomers and transition structures are shown in table 5. Calculated exothermicities and activation energies, obtained at a number of theoretical levels, are given in table 6; the MP3/6-311G***//MP3/6_3lG** results, corrected for zero-point vibrations, have been used to construct the schematic reaction profiles Table 5 Vibrationalfrequencies‘) and zero-pointvibrationaleneraies *) far speciesinvolvedin the HNC-+HCN and HOC’+ HCO+ rearrangements SpeCieS

Vibrational frequencies (cm-‘)

Zem-point energy &.I mol-‘)

HNC+HCN HNC T.s HCN

795 2111 960

795 2801 960

2161

3906

2254

3556

45.8 29.4 46.2

HOC++HCO+ HOC+ l-S HCO’

620 2056 1051

620 2208 1051

1997

3463

2288

3317

‘)

40.1 25.5 46.1

Calculatedat the RI%/4-31G//MP3/6-3?G’+level.

An observable

interstellar

species?

displayed in figs. 1 and 2. An examination of table 6 reveals the following: (i) The best previous estimates of the exothermicity of the HNC+ HCN rearrangement are 61 [5], 63 [38] 2nd 63 [37] kJ mol-‘, in excellent agreement with our caIculated MP3/6-3llG***//MP3/6_31G** value (61 kJ mol-I). This relative energy has been estimated experimentally [43] to be >45 kJ mol-‘. Similarly, our caiculated barrier height before correction for zero-point vibration (144 kJ mol-‘) is in excellent agreement with previous estimates of 146 [S] and 140 [37] W mol-‘. (ii) Inclusion of zero-point vibrational corrections lowers the barrier for each rearrangement by =15 kJmo!-*. (iii) The calculated exotherrnicities for each rearrangement are sensitive to the inclusion of polarization functions and electron correlation in the treatment. In comparison with the Mp3 results, RH!? underestimates the energy difference between the two isomers in each rearrangement whereas MP2 overestimates this diEerence. (iv) The barriers listed in table 6 for both rearrangements are substantially lowered by the inclusion of polarization functions in the basis set and are lowered further but to a smaller extent by the incorporation of electron correlation. The greater part of this effect may be attributed to the variation with level of theory of the relative energies of the less stable isomers so that if the barriers are calcuiated relative to the more stable isomer for each rearrangement, the values obtained for leve!s highhar than FUFIF/4-3lG//RHF/4-31G are relatively constant. (v) Although there are significant differences between the RHF/4-31G, RHF/6-31G** and MP3/6-31G** geometries, we note that RHF/6-31G**//I2I-IF/4-31G, RHF/631G**//RHF/6-31G** and RHF/631G**//MP3/6_31G** relative energies are in good agreement, as are the MP2/631G**//RKF/4-31G and MP2/631G**//MP3/6-3lG** and the Mp3/6SlG**//RHF/4-31G and MP3/6-

5

R.H. Nobes, L. Radom / HOC’: An observable inrersrellarspecies?

6

Table 6 Exothermicities and activation energies fcr the HNC+HCN and HOC+-rHCO+ rearrangements HNC+HCN ’

RHF~4-31G~/RHF~4-31G RI-G&31G**/iRHF/4-31G

RHFj6-31G**//RHFl6lG** RHF/6-31G*+//Ml’3/6-31G** RHF/6_311G***j/MP3/6_31G*+ MP2/6-31G**//RHF/4_31G MP2/6-31G**//MP3/6-3lG** MP2/6-311G***//,MP3/6-31G:‘f MP3/6-3lG**//RHF/4-3lG MP3/6-31G*+jlMP3/6-31G+* MP3/6-31tG*“//MP3/6-31G:‘i corrected c’

HOC++HCO+ ‘)

exothermicity (kJ mol-‘)

barrier (W mol-I)

exothermicity (kJ mol-‘1

40 46 46 44 44 80 83 75 66 67 61 61

241 162 165 163 15.5 145 146 143 147 148 144 127

123 123 120 136 181 185 196 151 152 163 157

91

barrier (16 mol-‘1 322 227 229 225 208 161 158 145 183 179 165

1.50

‘) Energies

calculated relative to the following total energies for HNC: -92.85542 (RHF/6-31G**//RHF/4-31G), -92.85851 (RHF/6-31G**//MP3/6-31G**), -92.88400 (RHF/6-311G***//MP3/6-31G**), -93.13265 (MP2/6-31G**//RHF/4-31G), -93.13358 WP2/6-31G**//MP3/6-31G**), -93.19024 (MP2/6-3llG***//MP3;6-31GI*). -93.14025 (MP3/6-3lG**//RHF/4_31G), -93.19536 (MP3/6311G***//MP3/6-31G**). ” Energies calculated relative to the following total energies for HOC*: -112.92073 (RHF/6-31G**//RHF/4-31G). -112.92051 (RHF/6-3lG**//MP3:6-3lG**), -112.94937 (RHF/6-311G***//MP3/6_31G**), -113.19283 (MP2/631G**//Rm/4-31G), -113.19315 (h4P2/6-3lG**//MP3/6-3lG**), -113.26277 (MP2/6-311G***//MP3/6-31G**), -113.19923 (MP3/6-XG*‘//RHFf4-3lG), -113.26704(MP3:6-311G***//MP3/6_3lG**). =’MP3/6-311G***//MP3/6-31G** values corrected for zero-point vibrational energy.

31G**//iMP3/6-31G** results, i.e. relative energies for the systems examined here are quite insensitive to the level of geometry optimization employed. (vi) HOC’ is substantially higher in energy (157 kJmol-‘) than HCO’. On the other hand, HNC is only 61 Id mol-’ higher than HCN. (vii) The calculated barrier to rearrangement of HOC+ to HCO’ is quite high (150 kJ mol-‘j and comparable to the barrier in the HNC/HCN system (127 k.Tmol-‘). Rearrangement of HOC+ to the more stable formyl ion is therefore predicted not to be a facile process.

tion (1): H;+CO-+HCO+i-Hz,

H;+OC+HOC’+HZ.

of HOC’

Based on relative abundances of various species occurring in interstellar clouds, Herbst and KIemperer [12, 261 proposed that formation of the formyi ion occurs primarily via reac-

(2)

We have performed calculations allowing Hz of to approach either the carbon- or oxygen-end CO along the molecular axis. Loosely-bound complexes of Hz with either HCOi or HOC’ result: H; + CO -, Hz. - -HCO+,

3.3. Fmnation

(1)

and that HOC+ might be produced in a similar manner:

(3)

H;+OC+Hr..HOC+. We find no barrier to either of reactions (3) or (4). The Hz- - -HCO’ complex has been encountered previously r44-481 in studies of the CH# potential energy surface. We have%lso

7

R.H. Nobes. L. Radom / HOC’: An observable inrersteliarspecies?

studied dissociation of these complexes into molecular hydrogen and the corresponding ion: HZ- - -HCO+ -+ HZ+ HCO’,

(5)

H,.~.HOC+-,Hz+HOC’.

(6)

In each case the energy is found to increase monotonically as dissociation proceeds. RHF/4-31G

optimized geometries of the

complexes Hz--.HCO+ (I) and HZ_--HOC’ (2) are shown in scheme 1. MP3/6-31G** energies

Ho-734

Fig. 1. Schematic reaction profile for rearrangement of to HCN including zero-point vibrational contributions.

----___

I.082

-a-H-C---O

I “__-&--

_l-098

HNC

H-._ Cl.738

I

+

-_

O-998 I-158 -\ *- :H-O---_-C

Hc-&& 2 Scheme 1

Fig. 2. Schematic reactionprofile for rearrangement of HOC+ to HCO+ including zero-point vibrdional contributions.

and RHF/4-31G vibrational frequencies for species involved in reactions (3x6) have been calculated; the results are shown in table 7 along with zero-point vibrational energies. In each case the RHF/4-31G geometry has been used (H:, r,=O.845 A: CO, r,= 1.128 A; HZ, r, = 0.730 A). MP3/6-31G”* relative energies have been corrected for zero-point effects to yield the reaction profiles shoti in fig. 3. Examination of fig. 3 reveals the following: (i) Formation of both HCO’ and HOC” are exothermic processes with no detectable potential barriers. Reaction (2) is thus predicted to provide a viable means of HOC’ production in the interstellar medium. (ii) The excess energy of HOC’ produced vi2 reaction (2) (41 kJ mol-‘1 is substantially less

8

R.H. Nobes, L. Radom / HOC’: An observable inrersiellar species?

Table 7 Mp3/6_31G**//RHF/4-31G total energies and RHF/4-31G//RHF/4-31G species invo!ved in possible formation reactions for HCO’ and HOC* Species

Total energy

Vibrational

HZ CO Hz-. -HCO-

-1.33014 -113.01834 -114.42383

H?...HOC+

-114.37319

HZ HCO’ HOC+

-1.16314 -113.25670 -113.19923

2935 2304 135 1073 165 818 4645 1012 497

frequencies

3810

140 2371 172 2015

224 3371 373 3160

478 4.546 636 4500

1012 497

2383 2006

3437 3630

3. Schematic reaction profiles for HCO’ and HOC’.

frequencies

and zero-point

energiestor

Zero-point energy (kJ mo1-‘1

(cm-‘)

2935

than that required to surmount the barrier to rearrangement to HCO* (150 kJ noI-*). Thus HOC’ produced via (2) should be stable with respect to intramolecular rearrangement. (iii) HZ...HOCi is predicted to be more tightly bound than is Hr - -HCO’ (20 k.Tmol-’ versus 5 kJ mol-‘). Experimental studies of the protonation of CO by Hz have been carried out by Tanner et

&

vibrational

1072

57.9 13.8 80.2

772

75.4 27-8 46.9 39.7

al. [49, 501 using the flowing afterglow technique. These experiments failed to p;roduce any evidence for the formation of HOC during the protonation process. However, under the prevailing experimental conditions, any HOC’ that might have been formed could react with the parent CO molecule: CO+HOC+-,HCO++CO.

formation

including zero-point

(7)

vibrational

contributions.

R.H. Nobes, L. Radom / HOC+: An obsemable interstellarspecies?

Our calculations (table 6) show that reaction (7) is exothermic by = 157 W mol-‘. Moreover, additional calculations at the RHF/4-3lG level indicate that reaction of HOC” with CO proceeds without actioation energy to yield a complex (3) of CO with HCO+ (see scheme 2). This

this work, and to Dr. Willem Bouma for helpful comments on the manuscript.

References Cl] L.E.

l-:42 l-664 c-_-*-____--&--~--~+

I.109

9

Snyder

and D.

Bchl,

Astrophys

J. 163

(1971)

I-100

3 Scheme2 complex has been previously studied by Ikuta [Sl]. Once formed, the complex would have sufficient excess energy ( ~226 kJ mol-‘1 to diisociate to HCO’ plus CO in a reaction which is endothermic by 69 kJ mol-‘. Thus CO is an efficient catalyst for the conversion of HOC; to HCO+. We conclude that in designing laboratory experiments to observe HOC’, it would be important to exclude CO from the experimental system.

4. Conclusions Reaction of H: and CO to yield Hz and HOC’ is predicted to be exothermic (by 41 kJ mol-I) and to have no significant activation barrier. Although HOC+ lies substantially higher (157 kJ mol-‘) in energy than HCO+, the barrier to rearrangement to the more stable ion is large (150 kJ mol-‘). HOC” is therefore predicted to be a likely candidate for intersteilar observation. Our predicted equilibrium structure of HOC* has r&Z-O) = 1.155 * 0.003 A and r,(O-H) = 0.988~0_003 A, and is considered to be the most reliabie structure currently availatile for this ion. The predicted .7 = 1 + 0 rotational transition frequency of H’601’C” is 89.Oi

0.8 GE-k

Acknowledgement We are indebted to Dr. William Rodwell for many helpful discussions during the course of

[2]

k!::

[5]

PK.

Snydei and D. Buhl, Bull. Am. Astron. Sot. 3 (1971) 388. [3] W. Klemperer, Nature 227 (1970) 1230. [4] P.K. Pearson, G.L. Blec!cm2n, H.F. Schaefer III, B. Roos and U. Wahlgren, Astrophys. I. 184 (1973) L19. Pearson,

H.F.

Schzefer

.m and U. Wahlgren,

J.

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