The heI photoelectron spectra of the halogen azides, XN3(X = Cl and Br) and the halogen isocyanates, XNCO(X = Cl, Br and I)

Chemical Physics 47 (1980) 11l-124 0 North-Holland Publishing Company

THE He1 PHOTOELECTRON SPECTRA OF THE HALOGEN AZIDES, ISOCYANATES, XNCO(X = Cl, Br AND I) AND THE HALOGEN D.C.

FROST,

Department

C-B. MACDONALD,

of Chemistr~~, Unkersity

C.A. MCDOWELL

and N.P.C.

XNJX

= Cl AND Br)

WESTWOOD

of British Columbia, 1075 Wesbrook Mall, Vancomer,

B.C., Canada

MT1

W.5

Received 22 August 1979; in final form Iti November 1979

High yield routes to rhe unstable halogen azides and isocyanates have permitted vacuum ultraviolet photoelectron spectra to be obtained for the chlorine and bromine azides, and the chlorine, bromine and iodine isocyanates. The results are compared with ab initio and semi-empirical calculations, leading to a reassignment of the photoelectron spectra ofthe parent acids, HN, and HNCO in the high energy region. The halogen azide and isocyanate photoelectron’spectra provide an interesting investigation into how the orbitals of a linear pseudohalide grouping are perturbed by an otT-axis haiogen atom. A photoelectron spectrum for the unknown molecule FNCO is predicted.

1. Introduction

An off-axis atom attached to a linear or quasilinear triatomic azide or isocyanate grouping will confer important structural and spectroscopic properties to the resulting niolecule. Such an off-axis atom will cause the initially degenerate bonding and degenerate antibonding s orbitals of the linear molecule (C,,) to split into two pairs (bonding and antibonding) of in-plane and out-of-plane orbitals in the bent molecule (C,). The magnitude of the resultant separation for each pair of orbitals will depend upon the off-axis angle and the interaction of the off-axis atom with the linear grouping through mixing of orbitals of the appropriate symmetry. A particularly good example of such molecules is provided by the halogen azides (XN,) and the halogen isocyanates (XNCO), where the perturbation of different halogen substituents upon the quasilinear pseudohalide groupings can be investigated. It is anticipated that the halogen p orbitals will interact strongly with the n and c framework. The halogen pseudohalides have not been extensively investigated previously, since the azides are extremely explosive and the isocyanates have only recently been prepared as monomers [I]-

There is, however, some recent 15N NMR and ultraviolet spectroscopic work on solutions of ClN, [2,3] and IN3 [G]. The BrN, and INCO molecules have never been previously characterized in the gas phase, although they are both known in solution [5]. The gas phase structures of ClNCO and ClN3 have been determined, however, and interestingly, it has been shown both by electron diffraction [6] and microwave [7] spectroscopy that the CINC bond angle in ClNCO is less than 120”, compared to values around 140” for CH,NCO and NCNCO, and about 128” for the acid, HNCO [S]. In the case of the azide, CINX, the bond angle from microwave spectroscopy is 108.7” [9] compared to angles in the range 114-120’ for CH,N,, NCN, and HN, [S]. Thus the bond angles for the chlorine substituted pseudohalides are signi!icantly less than those of the other substituted pseudohalides. Further structural relinements indicate the possibility of slight deviations from linearity (c 10”) in the azide and isocyanate triatomic groupings [6,7,9]. The photoelectron (PE) spectra of the parent acids, HN, [lO-14] and HNCO [lo, 11, 15]‘have been extensively investigated, together with those of the methyl, silyl and germyl substituted analogues [1 I]. There is considerable discord on the proposed

111

D.C. Frost er cd./PI~oroeIecnon specrru of

assignments, and this will be considered later. Other small substituted nzides and isocyanates that have been studied by PES include cyanogen uzide NCN, 116, 171. cyanogcn isocyanatc NCNCO 1163 and vinyl isocyanate CH,=CHNCO [IS]. Thus. continuing our previous work on unstable azides and isocyanates 1163 we now present.HeI PE spectra of chlorine azide, bromine azide. chlorine isocyclnatc, bromine isocyanate and iodine isocynnate. We wish to investigate the effect of halogen substitution upon the pseudohalide grouping, and also use the results to provide a better understanding of the electronic structures of the parent acids. To this end we have also performed some semi-cmpirica1 (HAM/3 [19]. CNDOjBW [ZO])_ and minima1 basis set ab initio [Zl] calculations. We also have recourse to some previous ab initi’o calculations on CINCO and CINJ [z]. Additionally, we would like to indicate how pure samples of these compounds may be prepared for furure gas phase investigations. We have not studied IN, or FNS since these molecules arc extremely hazardous’. FNCO is at present udcnown although it has been suggested as an unstable intermediate [23]. An excellent review is recommended [5]_

2. Experimental Tbc bromine and chlorine azides were prepared by the pnssage at low pressure (ca. 1-3 Torr), of the appropriate halogen through a 15 cm column IooscIy packed with moist NaN, supported on glass wool. These are essentially adaptions of the original methods of Spencer [X6]. and more latterly those of Millignn and Jacox [27] and CIark and Clyne [XI. With the NaN, maintained at room temperature N2 alone was observed indicating complete

’ The s:r?biIitv appears to increxe slightly in the order FN, c INir< BrN, -z GIN,. For those wishing zcIrcmpt such preparations by methods analogous those described below. we wish to point out that likely to condense in the reaction vessel with the precursor, and as such is liable to detonate [24]. rather fickle [x]. and involves the use of gaseous NzFr and NF, are likely to be considrrable sideproducts.

to to IN, is NaNJ

FN, is F2_

Izulogrr? oxides

and isoc~‘anno~s

decomposition of the azide. Clean spectra of pure GIN, and BrN, can only be obtained by holding the NuNJ tube at WC, and maintaining a slow flow of the halogen. No attempt was made to trap the azide, but a following trap filled with CaCl, and held at O’C sufliced to remove excess H20. No explosions were obtained with this procedure. Careful control of the halogen flow rate is important. At low pumping speeds aii the halogen reacts to form the azide, but this then undergoes some subsequent decomposition. Faster pumping rates improve the yield of azide, but some water and unreacted halogen are observed. In this respect the packing of the column and the following trap is crucial. ChIorine isocyanate. CINCO. was prepared by the pyrolytic decomposition of trichloroisocyanuric acid (Eastman Chemicals) [29-321. (CINCO), was held at = ZOO’C and pyrolyzed at z 3OO’C on line to the spectrometer. Initially, some Cl, was observed but this disappeared after the first few hours. Under these conditions there was no evidence for further decomposition to give carbony isocyanate, CO(NC0)2 [29,31]. We have also found that CINCO can be generated directly by reaction of gaseous Cl, with solid AgNCO at 15O’C as outlined for the other isocyanates. BrNCO was prepared by the passage of Br2 at low pressure (1-3 Torr) over heated AgNCO at I5O’C [33]_ AgNCO was prepared as a white solid by precipitation from aqueous solutions of KNCO and AgNO, [34]_ It was thoroughly washed, dried and degassed for several hours under high vacuum at 15O’C before use. This is an important step since we have found that BrNCO is very susceptible to hydrolysis. An alternative preparative procedure involving the reaction of Br2 with (ClNCO), was also attempted [35,36-j. This turned out to be an excellent source of BrCl (PE spectrum recently reported [37]) but depolymerization of the (BrNC0)3 trimer proved diflicult and complex. INCO has only previously been prepared in solution [38], but we report that it can be generated in the gas phase by an analogous procedure to that described above for BrNCO. I, is passed at low pressure over heated (and dried) AgNCO at 5 19O’C. It is even more susceptible to hydrolysis than BrNCO, and appears to disproportionate to I,

113

D.C. Frost et ol./Plzotoelectron spectra of lzalogen azides and isocynnates

and a solid material. Rapid pumping into the spectrometer was necessary to obtain a reasonable INCO/I, ratio. Spectra completely free of I, were never obtained. In each case discussed above, the products from the reaction zone were pilmped directly into the photoelectron spectrometer, the spectra monitored, and the reaction conditions adjusted to give maximum yields of the desired products. The PE spectrometer has been previousIy described [39]. Calibration was effected using the known ionization potentials (IP’s) of Mel, HNCO, HzO, CO?, Nz and Ar. Resolution was variable, but generally in the range 25-40 meV.

3. Results

cl

GIN,-

n

Fig. I. The HeI photoelectron spectra of(a) GIN, and (b) BrN,. The latter shows a trace of Brz and N2 (!abelled).

The HeI PE spectra of CIN3 and BrN, are shown in figs. la and I b, respectively. That of GIN, is quite clean, and that of BrN, shows a trace of Brl and Nz. It is to be noted that the fifth IP of BrN, lies directly ‘under that relating to the ‘Xi state of Nz. Expansions of the first IP’s ofthese two molecules are shown in fig. 4. Figs. 2a, 2b and 2c show full scale

a”(b), a’(b). These are partly localized on the pseudohalide grouping. Two additional IP’s will arise from in-plane (a’) and out-of-plane (a”) IL orbitals with partial localization on the halogen atom. As will become evident later there is considerable mixing

PE spectra of the corresponding isocyanates, CINCO, BrNCO and INCO. The spectrum of CINCO also shows Ar (for calibration purposes), BrNCO shows a trace of CO?, and INCO indicates some residual I?_ This spectrum represenis the best ratio of INCO/‘II that was obtained. All the other molecules could be obtained virtually free of contaminants. Figs. ?a, 5b and SC show expansions of the first and second bands for all the isocyanates. The experimentally determined IP’s are given in tables I and 2 for the azides and isocyanates respectively. The observed vibrational structure is also inciuded,

together

with the proposed

BrNCO

b

assign-

ments and these will be discussed in .the subsequent text.

4_ Assignments IO Within the He1 energy region (21.22 eV), the non-bonding and bonding TClevels (of the originally linear molecule) will give rise to four IF% in the non-linear molecule, viz. a”(n.b.), a’(n.b.) and

12

IONIZATION

I4

16

POTENTIAL

18 20 teV1

Fig. 7. The HeI photoelectron spectra of(a) CINCO (plus Ar for calibration purposes), (b) BrNCO (trace CO1 labelled), and (c) INCO (I1 labelled and dotted lines).

114

D-C. Frost et oL/PlroroelecrrQn spectra of halogen azides and isocyanates

Table I Experimenral

IP’s and assignments

Band

for the halogen

azides”

GIN,

BrN, -v’(cm-‘)

IP (eV)

Assignment”

IP (eV)

Y’(cm - l) -

( I0.20)b 1 2 3 4 5

10.38 I’00 13.39 13.39 15.96

6 7

17.39 ,y.53

730 t 60

710 + 60

( I O.KJ)b’ 10.08 11.38 12.44 12.75 15.62

2100 t 60 12YO + 60

1970 I so 1940 + so

3a” 8a’ 2a” 7a’ 6a’ pli 1a” 5a’

17-S”’

‘iertical

IP‘s (adiabatic IP’s in parcnthcscs). All bands kO.01 eV except the first (20.01 b’The true adiabatic IP mav lie one quantum lower. =’ See text. valence orbital numbering. d’ Possible position for additional IP. See text for discussion.

eV) and bands 6 and 7 (~0.05

eV).

Table Z Experimental Band

IFS and assignments

for the halogen

CINCO

isocyanat&

BrNCO

INCO

Assignmer&’

-~ IP (eV)

liIcm_‘)

I P (CV)

I*’ (cm - ‘)

IP (eV)

I” (cm - ‘)

IlOOand 520 f 40

(9.89) 9.89 10.58 12.16 12.53 16.39 15.60

420 and 2070 f 40

----

.:

(10.72) IO.56 Il.64 13.87 13.87 17.59 16.32

7 8

18.61 202

I 2 3 4

IllOand 600 + 40

( 10.46) IO.46 11.20

13.01 13.21 17.12 15.98

f SO 950 910 or 1900 f 50 1100 + 80

IS.15 19.8

=’ Vertical 1P’s (adiabatic IFS in parentheses). ” See text, valence orbital numbering.

All bands

between orbitals of the same symmetry, and so the above terminology is only used as a qualitative guide. In addition pi type orbit& are e.upected to occur within the 21.7 eV range, giving it total of seven or eight observabIe IF?. The first wo IP‘s in all five molecules are therefore assigned to the non-bonding out-of-plane (3a”), and the non-bonding in-plane @a’) orbitals respectiveiy. This follows previous results for HN, [10-14-j.

HNCO

[IO,

I!,

15-j, NCN,

[16.

171 and

700 to 850 750 + 80

50.02

5a’ pa 4a’

17.55 (19.0)

eV except the first (+O.Ol

3a” 8a’ 2a” fd 6;1’ 1a”

eV) and 12st (f0.1

eV).

NCNCO [16], although there are signilicant changes in the structural features and overell band envelopes due to extensive incorporation of halogen p character (in particular, N-X anti-bonding character). The third and fourth bands are ascribed to orbitals involving some degree of halogen p orbital character, (?a”, 7a’) although they cannot strictly be termed “lone pairs” because of the considerable mixing with the pseudohalide grouping. They are N-X bonding in character. In both GIN, and ClNCO these two

115

D.C. Frost et al./Photoeiectron spectra o/halogen azides and isocyanares ire superimposed at about 13.4 and 13.9 eV respectively, but the relative intensities are indicative of two IP’s at these. positions. The acciden’d degeneracy is removed for BrN,, BrNCO and MC0 since the bromine and iodine valence state IP’s are closer in energy to the non-bonding x levels, and therefore interact more strongly. It is interesting to note that in the isocyanate series the magnitude of the splitting of these orbitals (Cl = 0, Br = 0.17 and I = 0.37 ev) is proportional to the atomic spin-orbit splitting values (<(Cl) = 0.07, 5(Br) = 0.3 1, i(I) = 0.63 eV) [a]. There are no degeqeracies involved here, and therefore rigorously there should be no spin-orbit splitting, although one may invoke local cylindrical symmetry about the N-X bond [41]. The main argument against this approach however, is that the Xp orbitals are not localized on X, and as we shall see later the first two IP’s in INCO actually have more iodine 5p character than the third and fourth IP’s. The first four IP’s for all five molecules are therefore easily assigned by comparison with known molecules. The overall scheme is summarized in fig. 3, which gives a representation of the orbital orbitals

energies as a function of the angular variation of a typic& XNCO molecule: This is, in essence, a Walsh diagram and has been constructed using. eigenvalues from an ab initio calculation on ClNCO with an STO-3G minimal basis set [Zl]. Schematic representations of orbital character are also included. For clarity s orbitals are not shown, although all a’ levels involve some s character_ Beyond i5 eV all five molecules show three or four additional bands, and heie the assignments become more trouble-some. The ab initio calculations are not entirely useful since the z bonding and the pa orbitals are predicted to lie close in energy (fig. 3). In addition, low lying unoccupied orbitals are expected to cause large corrections to theoretical values calculated using Koopmans’ theorem [42-M]. The problem is also compounded for the azides because there are a variety of interpretations relating to the PE spectrum of the parent acid, HN, [l&-14]. The main discrepancies involve the assignment of one or two IP’s to the fourth band, and the magnitude of the separation (and relative ordering) of the n bonding orbitals. A similar situation occurs for HNCO [lo, 11, 151, where the relative positions of the rc bonding and po (essentially oxygen lone pair) Cl-N=C*O orbitals are also in doubt. We therefore defer assignment of the last three bands in the azide series, t-&b and the last four bands in the isocyanates until the latter part of the discussion.

s

CINCO

-

.

-n 5. Discussion

-15. S.I. 3a” and Sa’ non-bonding

1, 100

‘. 120

CI -N -C

”

”

140 160 ANGLE

J

180

Fig. 3. Walsh type diagram Tar a bent and linear CINCO molecule (linear NC0 grouping) constructed with STO-3G eigenvalues (ordinate). Broken vertical line indicates experimental CINC angle. See text for proposed assignment above 15 eV. For clarity all s orbitals are omitted from the schematic orbital diagrams, although all a’ orbit& show considerable s character. No schematic is given for the da’ orbital, which is mainly s in character.

rt orbitals

In the unsubstituted HN3 and HNCO molecules, the Franck-Condon envelopes of the first two bands illustrate the out-of-plane and in-plane non-bonding character of these orbitals. Thus the first band has a single intense transition, with weak associated vibrational structure, whereas the second band is broad, consistent with a large change in geometry upon forming the ion (opening of the HN,N, (or HNC) angle). This latter orbital is stabilized since it is partially localized around N, and possesses s character (sp’ hjbrid). The separation considerable between the first two II% in HN, and HNCO is

116

D.C. Frost ef al./Phororlecrron spectra of Aalogen azides and isocJanares

1.52 and 0.79 eV respectively, the difference reflecting the bond angles, 114; and 125” respectively [S], and [he corresponding increase in s character. Substitution of a halogen atom for the hydrogen atom causes the off-axis angle to increase, such that all Xt$ and XNCO molecules are more bent than the corresponding acids. In terms of the orbiral energies (assuming Koopmans- theorem [42]_ see tables I and 2) this has a destabilizing effect due to an X-N antibonding interaction, and usually increases AE, the separation between the 7c nonbonding levels (3a” and Sa’). Thus ior GIN,. AE = 1.63 eV. whrrtis the parent acid value is I.52 cV. For CINCO, AE = 0.78 eV, the value for the acid being similar. The differences between the magnitude of AE for GIN, and CINCO are mirrored in the larger ofL_axisangle which obtains in the nzidc The smaller angle in both molecules compared to the parent acids. also has the effect of placing more s character at N,. and so the destabilization incurred by the antibonding halogen is parfially offset by a stabilization due to this increased s character. However, an additional Factor controlling the magnitude of AE is the raonance interaction between the Sa’ non-bonding z level and the 7a’ halogen based orbital, which forces the 3,” and 8a’ non-bonding levels together. This is more pronounced for BrN:. BrNCO and INCO, where the ia’ bromine and iodine p based orbitals are closer in energy IO the Sa’ non-bonding orbital, reducing the values of AE in BrN, (1.30 eV), BrNCO (0.74 eV) :rnd INCO (0.68 cV) to below the parent acid values. In addition. this resonance interaction causes the ?a” :~nd 7rr’orbit&. which in GIN, and CINCO are cffcctivcly degenerate, to separate, viz. BrNS (0.31 eV). BrNCO (0.17 eV) and INCO (0.37 eV). The diffcrenr values For the two bromo-compounds ;Lruuc against a spin-orbit splitting mechanism. If rhc supararion of these two orbitals is added to the AE values for the first two IP’s to take into account this additional resonance interaction. the final values kW) obtained for BrN, (I.61 eV), BrNCO (0.91 eV) and INCO (I.05 c-V) are all gredter than the parent z~cid vt~lucs as initially expected for molecules with %~~ller bond angles. Since structural data are unknown for the unstable bromo- and iodo-molecules, the values AE GIN provide an estimate of the structures. AE’ for

BrN, is now identical to that for’ClN;‘suggesting that they have similar angles @a. 109’). This is not unexpected by comparison with the structurally related nitrosyl halides, GINO and BrNO [453 which also have similar bond angles (= 114”). For the chloro- and bromo-isocydnates, the maximum values become virtually identical (A6 = ca. 0.91 eV). although the value for INCO is somewhat larger (AE’ = 1.05 eV). En the absence of other interactions this would imply that the bond angle for INCO is smaller than that for the chloro- and bromoisocyanates which have similar bond angles of about 119’. Relative to the parent acid, HN,, the first band of the halogen azides has ROW become broad (lig. 4) with associated vibrational structure, and the second band is relatively more intense. On the first IP of both GIN, and BrN, (vertical, 10.38 and 10.08 eV, respectively), live or six vibrational intervals are resolvable, with average vibrational spacings of 730 f 60 and 710 t 60 cm-’ respectively. There are no definitive vibrational assignments for the ground states of these two molecules, but assuming excitation of only A’ modes of which there are live, the possibilities reduce to vl., the N-X stretching frequency and 1~~.the in-pianc NNN bend (27,2S, 461. As it turns out, vN-Br and vN-CI in the molecular ground state are surprisingly close to

Fig 4. Expansion electron spectrum

of the first band in the He1 photoof (a) GIN, and (b) BrN,.

D.C. Frost et al./Photoelectror;

spectra ofhalogen

each other (687 and 723 cm- ‘, matrix IR [27]), and to the observ:d ionic vaIues indicating invoIvement of halogen p orbital character. The other possibility is a reduced value of v, NNN (va = 1160 and 1144 cm- 1 for BrN, and GIN,, respectively [27]). From a comparison with the observed structure on the corresponding isocyanates (below), we favour the latter alternative. In addition, a similar proportional reduction (1265 to 850 [13] or 980 cm-l [12]) is observed on the fust band of HN3. The three halogen isocyanare molecules also provide an interesting series, since from expansions of the spectra (lig. 5) it is evident that the first band comprises at least two distinct vibrational progressions. In CINCO there is a main progression of 1110 k 40 cm” extending over at least live members, together with some weaker structure with a spacing of 600 + 40 cm-‘. The vertical transition occurs at D’= 1 (10.86 eV). Similar structure is observed on the first band of BrNCO, although here the maximum occurs at U’=i 0 (10.46 eV), and the

I

C

I

120 &NC0

IO5

II.5 r:i. r7

UK0

Fig. 5. Expansion of the iirst and second bands in the He1 photoelectron spectrum of (a) CINCO, (b) BrNCO (trace HNCO labelled) and (c) INCO. (Note the ‘n,,2 state of 1; lying under the first band.)

ozides and isocyonaks

main progression of 1 I00 + 40 cm-’ falls’offin intensity much more rapidIy. The weaker progression is 520 +_ 40 cm-‘. The band envelope in INCO is different, and more difficult to resolve due to the underlying 2111,Z state of residual I;. The vertical and adiabatic transitions are coincident at 9.89 eV, with a progression of 420 _+ 40 cm-’ extending over at least three members. A new progression involving an excitation of 2070 & 40 cm-’ is also observed. _ All three first IP’s of ClNCO, BrNCO and INCO show weak “hot” bands at 620,540 and 510 cm-‘, respectively from .the origin, corresponding to a smali population of the u” = 1 molecular ground srare level. From the known vibrational spectra of ClNCO [29,47] and BrNCO [33,36] it would appear that the predominant excitation of around 1100 cm-’ represents a reduced NC0 symmetric stretching frequency in the ion. Molecuiar ground state values for v1 are 1309 and 1296 cm-’ for ClNCO and &NC0 respectively. This reduction in frequency is about the same as that observed on the first band of HNCO (vi = 1050 cm-‘, UT= 1327 cm-‘) and follows our interpretation for the structure on the first band of the halogen azides. The weaker progression could possibly refer to vg, the NC0 in-plane bend: CINCO, v’ = 600 (v; = 708); BrNCO, 1” = 520 (v;’ = 689) and INCO, J = 420 cm --L_ However, in view of the downward trend in the ionic frequency from the chloro-through to the iodo-isocyanate, and the increasing relative intensity of this vibrational mode in the same direction, we assign it to excitation of Ye, the N-X stretching frequency. This would then be in accord with the considerable halogen character involved in this molecular orbital, which increases from the chlorothrough to the iodo-compound. In CINCO and BrNCO the observed values for these ionic frequencies are equal to, or greater thdn the molecular ground state values (v, = 603 and 473 cm- I, respectively) as expected for an orbital involving N-X antibonding character. For the INCO molecule no vibrational data below 1900 cm-’ is available [3S], but the observed value (420 cm-‘) is close to to the N-I stretching frequency observed for the trimer (a2 cm-‘) [S] and to the vaIue of 405 Cm-’ for vN-I in IN, [4-J The additional frequency of 2070 cm-’ observed on the first band pf INCO

118

D.C. Frost et al./PIzoroefecrran spectra of halogen azides and isocyonates

probably results from excitation of the NC0 asymmetric stretching frequency; v1 = 2180 cm- ’ in the molecular ground state [3S]_ An interesting and important feature for the isocyanate series concerns the observation that the first (and second) IP’s have an increasingly narrow bandwidth and a corresponding intensity increase in the direction CINCO through to INCO. In addition, the maxima for the chlorine substituted azides and isocyanates occur at u’ f I compared to the bromo- and iodo-compounds. Both these observations are indicative of increasing halogen character in the first two orbit& (3a” and Sa’) as one shifts towards the heavier halogen. This is not unexpected since the valence state IP of iodine (10.85 eV) [40] approaches the energy of the first two bands. It is therefore apparent that. particularly for the heavier molecules. one cannot distinguish between the x non-bonding levels (3a” and Sa’), and the halogen “lo’fie-pairs- (2a” and 7a’). To substantiate this conclusion we have performed some semiempirical SCF MO calculations (CNDO/BW) [ZO] for this complete series of isocyanates, although the results are not reproduced here in the interests of space. Known [7] and estimated geometries were used. These calculations confirm that the halogen character in the first two orbitals increases in the direction Cl to Br to I, such that the first two bands of INCO involve more iodine 5p atomic orbital contribution than the third and fourth bands. Similar conclusions are reached for the more limited azide series, where again the 3a” and Sa’ bands of BrN, are sharper than the corresponding bands of ClN,. The considerable interaction between the halogen p atomic orbita!s and the -NC0 and -NNN groupings implies considerable z character for the N-X bond in all these molecules, particularly for the azides where the orbital interaction is larger. This is in accord with the postulate of double bond character in the N-X bond of these molecules [s], as ilIustrated by the high N-X stretching frequencies in the azides, and the fact that vN-X does not increase appreciably from the azide XN, to the diatomic XN 1271. Similarly, the XNCO and XN, molecules have strong N-X bonds, e.g. r(N-CI) = 1.703 L%in CINCO, and r(N-Cl) = 1.745A in ClN,. This compares with other typical N-Cl bond lengths in the range 1.75 to 1.77 A [5].

5.2.2a” and 7a’ orbitals Although, as we have seen, these orbitals are extensively mixed with the 3a” and 8a’ orbitals, they do show the characteristic shift of halogen based orbitals, the average positions being 13.4 and 12.6 eV for chlorine and bromine azide, and 13.9, 13.1 and 12.35 eV for chlorine, bromine and iodine isocyanate. The separation of these orbitals has been noted earlier for BrN,, BrNCO and INCO, and supports the assignment of the intense third band in ClN, and ClNCO to two accidentally degenerate levels. In BrN, and INCO, residual traces of Br2 and I2 respectively, impart slight anomalies to the band envelopes (marked in figs. 1 and 3). It is interesting to note that, as observed for the monohalogenoacetylenes [4S], the ionization energies are a linear function of the ionization potentials of the free halogen atoms. This is illustrated in fig. 6 for the isocyanate series which gives a plot of --E versus the atomic ionization energies [1(X)] [40]_ The correlation is linear over the limited available data, the gradient of each line giving an indication of halogen involvement in the particular

Fig. 6. The experimental ionization energies of CINCO, BrNCO and INCO plotted against the ionization poten-

tials of the free halogen atoms. Prediction of the PE spectrum of FNCO.

D.C. Frost et al.JPhotoelectron spectra of halogen azides atid isocyanates Table 3 Predicted and calculated IP’s for FNCO”

Predicted from lig. 6b1

Calculated, HAM/3”

12.5 13.5 16.1 16.5 17.5 19.7 S.5

geometry Id1

geometry II”

11.96

11.60 (a”) 13.11 (a’)

” A11values in eV. ” Using atomic ionization

‘I Ref. [19].

(a”)

12.48 (a’) 15.51 (a”) 15.66 (a’) 17.20 (a’) po 17.90 (a’) 17.92 (a”) 20.56 (a’)

d’Ref. [49].

15.44(a’)

15.50 (a”) 17.25 (a’) 17.36 (a”)

18.04(a’)

pc

19.80 (a’)

ref. [40]. ClRef. [SO].

energies,

orbital. Extending this plot, we have projected a probabIe PE spectrum for the unknown molecule FNCO. TO substantiate this prediction we have also performed some semi-empirical calculations on FNCO using the HAM/3 programme [19], which we have found to perform with good accuracy in the calculation of IP’s for a wide range of molecules. The HAM/3 IF% for FNCO using a CNDO/Z computed geometry [49] and an ab initio geometry [SO] are given in table 3. Agreement within 1 eV of the extrapolated values is obtained for the first five IP’s. The two computed bond angles from the semiempirical [49J and ab initio [SO] calculations are 134.8’ and 113.8” respectively, interestingly both larger than the corresponding predicted bond angle for ClNCO. Unfortunately the HAM/3 procedure is not yet parameterized for chlorine, and SO calculations on the key molecules CLN, and CLNCO are not possible by this method. However, HAM/3 calculations have been performed on HNCO and HN3 [Sl, 523 in order to unravel the disagreements in the assignments for both these molecules above 15 eV. 5.3. pa(a’) and deeper levels eV all five molecules

show three or bands, and it is here that the isocyanates and azidcs may be expected to differ in relative ordering. The unkfiown factor involves the Beyond

15

four additional

!I9

magnitude (and direction) of the separation of the x bonding orbital (to give a” and a’levels), and thus the extent of the perturbation upon this orbital due to the halogen substiiuent. The correct ordering for the deeper orbitals of the parent acids can be established by an analysis of the calculations for HN, [12, 13, 51, 53-561 and HNCO [15,49, 51,53,X%1]. These include some recent calculations involving HAM/3 [Sl] and perturbation corrections to Koopmans’ theorem [52]; and indicate that the most plausible interpretation involves a scheme where the n-bonding levels are hardly spIit at all for both HN3 and HNCO, contrary to the results given in refs. [12-151, and that they constitute the fourth and fifth W’s in HN,. and the third and fourth IF% in HNCO. We should also point out that the x non-bonding separation (4-5 eV) for the isoelectronic Czv analogues, HICNN [13] and H&CO [62] is also much greater than the JCbonding separation (l-2 eV). A correiation of the orbital energies of H&NN with those of HN,, and HJJCO with HNCO further supports the proposition that the pa orbital in the PE spectrum of HNCO was misassigned [ 151 on the basis of some CND0/2 calculations which invariably place G orbitals too high in energy. For the halogen substituted pseudohalides the question remains whether we can ascertain the effect of an off-axis halogen substituent upon the x bonding separation and the position of the pci orbital. We have recourse to our own STO-3G calculations on GIN, and CINCO, and to some earlier ab initio calculations [22] on the same molecules. These results are presented in table 4. The Iifth IF’ in the PE spectra of the halogen azides corresponds directly to the third IP in HN, and is therefore unambiguously assigned to the 6a’(po) orbitai. It is slightly stabilized, 0.49 eV for GIN, and 0.15 eV for BrN, by comparison with the parent acid. This is probably due to the inclusion of some Cl 3s character, a conclusion substantiated by both ab initio calculations. This particular band in both halogen azides maintains the strong O-O transition observed in the parent acid and in NCN, [l6]. The resolved structure, 1940 cm- 1 for BrN,, and 2100 and 1280 cm-’ for ClN,, provides confirmatory evidence for localization in the -NNN

gouping

the observed values representing

the

D.C. Frost et al./Hrotoelectron specrra of halogen a:ides and isoc.samres

120

Table 4 Ab initio calcullltions ___-

For GIN, and ClNCO ~____ CINCO

GIN, --assianmcnf’ ja-

Sa’ 7ri a-

Kosmus et al.=’

STO-3cP

assignment”’

Kosmus et al.”

S-I-O-3Gb’

10.5s i2.56 14.01 14.47

8.94 10.70 12.17 12.73 16.93 1S.S? 19.35 19.33 27.82

32” 8a’ 7a’ 2n” 6a’ PG 1a*’ 5a’ LTn’ 3a’

11.74 12.54 15.04 15.33 18.84 19.30 19.64 21.07 30.46

10.22 12.76 13.24 16.68 17.51 17.04 18.70 27.70

6a’pc

18.90

5, ia” &Is 3a. ---

20.29 20.5 I 10.97 30.3 1

9.40

----

at Ref. [22]_ experimental geometry. ” This work. cxpcrimental seometrv. Note difkrences L’ Vulrnce orbirai numbering. -

for la” and 4a* levels of GIN,.

symmetric and antisymmetric NNN stretching frequencies. These bands are shown in detail in lig. 7 for GIN, and BrN,, the latter also showing the last t\vo !P’s relatively unhindered by the IF? of N2 (compare fig. 1). These remeining two bands are broad. and or low cross section, occurring at 17.04 and lS.01 eV in BrN,. and at 17.39 and 18.53 eV in GIN,. Lowe follow the results for HN, [IO] and XCN3 [16]. und the ab initio calculations for GIN, (tablr 4) then we conclude that these two bands in BrN, and GIN, are the la” and 5~’ ?r bonding urbitals. The actual ordering is problematical since the two ab initio calculations place these orbit& within 0.5 eV orench other. Intuitively we prefer to place the band at higher binding energy to the 5a’ orbital. since it should be stabilized by the inclusion

Fig 7. Expxkon sixth und seventh

or(a) the lifth IP of CLN, (6) the fifth, IP’s of BrN,. A trace of N, is labelled.

and the la” and ?a’ levels of CINCO.

of Cl 3s character. This is supported by an analysis oTthe eigenvectors which show only a small Cl 3p contribution to the la” orbital. Both bands in the photoelectron spectra are broad and featureless confirming the overall bonding nature of these orbitals. It should be noted that both ab initio calculations for ClN3 (table 4) place an additional orbital (?a’) very close in energy to the la” and 5%’levels. En fact. with the minimal basis STO-3G calculation this has virtually the same energy as the la” 7cbonding level. From the experimental spectrum there are no obvious additional IP’s to accommodate this predicted eighth orbital, although we cannot completely preclude the presence of a weak band masked by the last two bands (e.g. ca. 17.8 eV for GIN,). This orbital which is partly N-Cl bonding is expected to be weak in rhe He1 spectrum due to a large incorporation of nitrogen 2s and chlorine 3s character. inspection of the SCF output indicates that ClN, po&esses low lying 3’ (4.2 eV) and a” (5.8 eV) virtual orbitals, and from the RSPT results on HN, and HNCO [52] large Koopmans’ corrections to the lx orbitals are indicated. Assuming that the two observed bands provide a measure of the maximum splitting of the TCbonding orbitals. we obtain values of 1.:4 and 0.96 eV for GIN, and BrN,, respectively, both larger than in the parent acid, but still less than the x non-bonding separation.

121

D.C. Frost et al./Photoeiectron spectra of halogen atides and isocyanates

For the isocyanate molecules investigated here the situation is even less clear, because the last four bands in all three isocyanate molecules have different Franck-Condon envelopes from the corresponding bands in the azide molecules. Again the ab initio calculations on ClNCO (table 4) are particularly unhelpful, since both bunch the three IP’s to within 0.8 eV. Experimentally these bands span 2.3 eV. Additionally, an extremely broad, weak band occurs in the spectra of CINCO (20.2 eV) and BrNCO (19.8 eV). Following the ab initio calculations for ClNCO, (table 4 and fig. 3) we assign this to the 4a’ orbital which has some N-Cl bonding character, but is predominantly nitrogen, carbon and oxygen 2s, and chlorine 3s in character. Assessing. all of the available evidence, we assign the fifth and sixth IP‘s to the rt bonding orbitals, la” and 6a’ respectively, and the seventh IP to the 5a’ (PC) orbital. Placing the PG orbital as the fifth IP positions it too high in energy by comparison with HNCO and ClN,, and placing it between the 7~bonding orbitals would give too large a R bonding separation. The present assignment would give n bonding separations of 1.27, 1.14 and 0.79 eV for ClNCO, BrNCO and INCO respectively, in all cases larger than those for the corresponding azides despite the smaller off-axis angles. The reasons for this may lie in the heteroatomic nature of the -NC0 grouping which is expected to give a more uneven distribution of the individual atomic orbitals compared to the -NNN grouping. Thus from the calculations, the 6a’ orbital of ClNCO possesses very little s character on nitrogen compared to the comparable 5a’ orbital of GIN,, whereas the la” orbital shows some Cl 3p involvement in CINCO and virtually none in the same orbital of ClNs.

onset is sometimes obscured by residual COZ, but there is evidence for some structure, namely 750 f 80 cm-‘. Similarly, the same band in ClNCO has a sharp onset and a partially structured tail (ca. 1100 & 80 cm- *). The ab initio calculations differ on the relative ordering of the in-plane and out-of-plane R bonding orbitals (table 4) but as mentioned above, we prefer to assign the fifth IP to la” (least shift) and the sixth IP to 6a’ (stabilized by Cl 3s). The assignment of these last three bands is therefore by no means certain, but the available evidence that we have presented indicates that the a”ja’ bonding level separation is probably very small in the parent acids, ca. < 0.5 eV. Substitution of a halogen atom increases this separation to an average of ca. 1.05 eV for the chlorine and bromine azides, and at least 1.20 eV for the chlorine and bromine isocyanates. The value decreases from chlorine to iodine, that for INCO being only 0.79 eV. Since the assignment in this region is by no means definitive it is still open to reinterpretation. As we have mentioned before, part of the unsatisfactory correlation with the ab initio calculations stems from the fact that all these molecules possess iow-lying unoccupied orbitals and therefore large corrections to Koopmans’ calculated values are necessary. In addition, it is in this region that breakdown of the one band/One occur appear

orbital

relationship

[63]. The present

to give the best account

experimental

is expected

assignment

does

to

however,

of the available

results.

We suggest that a comprehensive analysis of this data may be obtained using the more elaborate methods of Green’s functions [64] or perturbation corrections to Koopmans’ theorem [43,44,65-J.

and

conclusions

The shape of these bands and the associated structure (table 2) are not particularly helpful. In

6. Summary

most cases the structure is weak, involving reduced NC0 symmetric and antisymmetric frequencies, simply indicating the bonding nature of these orbitals. However, the penultimate band in all three isocyanates (occurring at maxima of 18.61, 18.15 and 17.55 eV respectively in the chloro-, bromo- and iodo-isocyanates) is weak in intensity, which would be appropriate for the PG orbital with considerable s orbital involvement. For the bromo-compound the

The ultraviolet photoelectron spectra of the halogen azides, CINs and BrN,, and the isoelectronic halogen isocyanates, CINCO, BrNCO and INCO have been obtained and analyzed. Several of these molecules have never been characterized previously in the gas phase, and so this study opens the way to further gas phase spectroscopy of these molecules. Several interesting features emerge from this investigation which vve mention in concluding.

D.C.

112

Frost et

d/Photoelectron

spectra

(I) The photoelectron spectra of HN3 and HNCO have been reassigned, providing a basis for the propo’sed assignments of the XN, and XNCO molecules. However, inspection of the He11 spectra for HN, [l2-141, does indicate a weak peak at 21.6 eV that cannot be accounted for by a one electron model. Multielectron processes may be important here. (2) Figs. 8 and 9 provide a summary cf the results and indicate how the bent HNCO and HN3 molecules are derived from the linear CO2 and N1O molecules respectively, and how subsequent substitution by Cl, CH, and CN groups affects the orbital energies. (3) A Walsh diagram ior HNCO (not shown), can usefully be compared with that for CINCO (lig. 3). Comparison of the resultant curves indicates that the geometry of HNCO is determined mainly by the competing energy requirements of the 6a’ (n.b.), 4a’ (pt) and 3a’ orbitals. In the case of the chlorosubstituted molecule the three corresponding orbitals are all stabilized with an increasing bend at nitrogen, until at some point the PG level starts to rise in energy. On balance the sum of the orbital energies for CINCO indicates it to be more bent than the parent HNCO molecule. A similar result obtains for

I

t

co2

HNCO

CINCO

oi,NCO

‘.-.-’

NCNCO

I

of halogen

azides and isocyanares

GIN, versus HN,. The STO-3G calculations also show that the N, atom in GIN, has mores character, indicating 2 more bent structure compared to CINCO. Extension af this approach to CH, and SiH, substituted azides and isocyanates (which Favour a more linear arrangement) is proposed, aIthough the shallow potential functions involved will require a detailed basis set and CL study. In summary we are now able to follow the orbita energies through the entire series NCO” (or N;) HNCO (or HNJ and ClNCO (or CIN& Thus, bending the linear 16 valence electron ions NCOand N; removes the 7~degeneracies and stabilizes the out-of-plane orbital (a”) more than the in-plane orbital (a’). Addition of an off-axis H atom, and retention of a linear pseudohalide grouping to give the isoelectronic HNCO and HN, molecules will, however, stabilize the a’ orbital preferentially since this orbital has the required symmetry to interact with the H Is atomic orbital. The a” orbital remains localized on the triatomic grouping and thus it is found that H hdS a limited perturbing influence. We now find that an off-axis halogen atom, X, with directed p orbitals can affect the orbitals of the linear triatomic grouping in an entirely different manner, since the p orbitals of X have no symmetry restricN20

HN3

GIN,

CH,Ns

NCN,

I

Fig. 8. Correlation of the orbital ener& of COZ, HNCO, CINCO, CH,NCO and NCNCO. The results for CH,NCO and NCNCO are taken from refs. [I I] and [16]. respectively.

Fig. 9. Correlation of the orbital energies of NNO, HN,, ClN3, CH,N3 and NCN3. The results for CH,Na and NCN, are taken from rers. [l I] and [16], respectively.

D.C. Frost et nl./Photoelectron

spectra

tions in such low symmetry molecules, and can therefore interact intimately with the n and g system of the quasi-linear triatomic grouping. The net result is a substantial splitting of the z bonding pseudohalide orbitals, the extent of the perturbation depending upon the nature of the halogen (energy of the perturbing group, X). Thus we find that the interaction ofX with the non-bonding z levels increases in the direction, chlorine through to iodine, giving quite different Franck-Condon enveIopes and substantially different PE spectra from those of the parent acids. The interaction of X with the bonding ILorbital (which is effectively degenerate in the parent acids) indicates an increase in the separation from iodine through to chlorine. We are at present investigating such effects in other pseudohalide molecules. Acknowledgement The linancial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. We thank E. Lindholm for the HAM/3 programme, D.P. Chong for adapting it to our computer, and preprints of some papers, and G.D. Zeiss for several useful discussions.

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