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He I and Ne I photoelectron spectra of dichloro- and dibromomalononitrile
Journal of Molecular Structure, Elsevier
Science
Publishers
B.V.,
162 (1987)
341-350
Amsterdam
-
He I AND Ne I PHOTOELECTRON AND DIBROMOMALONONITRILE
C. B. MACDONALD,
D. S. WADDELL
Printed
in The Netherlands
SPECTRA OF DICHLORO-
and N. P. C. WESTWOOD*
Guelph-Waterloo Centre for Graduate Work in Chemistry, Guelph, Ontario Nl G 2 Wl (Canada) (Received
12 June
University of Guelph,
1987)
ABSTRACT He I and Ne I photoelectron spectra are reported for the densely populated valence regions of gaseous malononitrile, H,C(CN),, dichloromalononitrile, Cl,C(CN),, and dibromomalononitrile, Br,C(CN),. A minor reassessment of the H,C(CN), assignments is extended to permit plausible assignments for the previously unreported dihalodicyano species. Semiempirical calculations, HAM/3 and MNDO for H,C(CN),, and MNDO for the X,C(CN), molecules, are shown to be of limited value for the location of strongly localised nitrogen orbitals.
INTRODUCTION
The photoelectron (PE) spectra of many CN containing molecules are known, and are often complex due to the overlapping rcN and ncN orbitals in the 12-l 5 eV range [l]. In addition, even ab initio SCF calculations do not adequately represent the measured ionisation potentials (IPs) of such compounds due to severe Koopmans’ problems. Of relevance to this work is the parent Czv molecule, dicyanomethane (malononitrile) the PE spectrum of which has been recorded and assigned using a simple MO model based on through space and through bond interactions [2]. In this work we evaluate both the He I and Ne I PE spectra for malononitrile and the efficacy (or otherwise) of semiempirical (MNDO) and HAM/3 calculations. We then extend these results to the He I and Ne I PE spectra of the dihalo species X,C(CN),. Here, ionisation due to the halogen (X) lone pairs is expected to complicate the issue with a grand total of 10 (X + CN) valence IPs up to 15 eV. The overall Czv structures for these molecules are substantiated by the IR and Raman spectra of H,C(CN)* [3, 41, ClzC(CN)2 [5] and Br,C(CN), [5], in addition to the microwave spectrum [6] and crystallographic [7] studies
*Author
to whom
0022-2860/87/$03.50
correspondence 0 1987
should
be addressed.
Elsevier
Science
Publishers
B.V.
342
of H,C(CN),. This facilitates a combination of the assignments for dicyanomethane with the known assignments for the Czv dihalomethanes [S] for evaluating the spectra of the dihalodicyano molecules. EXPERIMENTAL
H,C(CN), (Eastman) was purified by recrystallisation from ether, and dried in vacua before use. Cl,C(CN), (Aldrich) was used without further purification. Br,C(CN), was prepared by addition of Brz to aqueous H,C(CN), as previously described [ 91. Photoelectron spectra were obtained on a specially constructed spectrometer with a resolution (under sample operating conditions) of 30 meV. The He I PE spectra were used to obtain accurate IPs, calibration being effected with the known IPs of Ar, N2 and CHJI. The Ne I light source is an approximate 5:l doublet at 16.85 and 16.67 eV. Thus, in order to obtain more meaningful relative intensities, ionisation due to the weaker line was digitally subtracted; this procedure was monitored using the band corresponding to the “Z:‘g state of N: in order to ensure fidelity. MNDO calculations were performed using the MOPAC package [lo]. HAM/3 calculations [ll] were also performed for H,C(CN), to assist with the assignments. The calculations were implemented on a VAX 11/750 computer. RESULTS
The He I PE spectra of H,C(CN),, Cl,C(CN), and Br,C(CN), are shown in Fig. 1, and the corrected Ne I spectra up to 15 eV are shown in Fig. 2. IPs and vibrational frequencies for H,C(CN), are listed in Table 1 along with the results of the present MNDO and HAM/3 calculations, and some multiple scattering MS X, calculations [12]. The third column shows the current assignments, which are discussed below. The IP values are in good agreement with those reported earlier [2]. Tables 2 and 3 show the measured IPs and proposed assignments for the dichloro- and dibromo- derivatives, respectively, together with the MNDO calculated values. The calculated (MNDO) structures for H,C(CN),, Cl,C(CN), and Br,C(CN),
N
N
N
That for H,C(CN), is in reasonable agreement with results obtained from microwave spectroscopy [ 61, including slight non-linearity in the CCN angle.
343
t
j\bL h----I 11
_._12
1
IONIZATION
POTENTIAL
Fig. 1. He I photoelectron
v
17
15
13
(eV)
spectra of H,C(CN),,
Cl,C(CN), and Br,C(CN),.
The results for the X,C(CN), molecules should provide a good starting point for structural investigations. In addition to those results shown in Table 1, some earlier MNDO results are available [13] for H,C(CN), but these are not included since, although some values correlate with our results, there are severe discrepancies/typographical errors in the numbers and orbital assignments, e.g. the results [13] show 4 X al, 4 X bl, and 1 X bz orbitals, whereas the nine available valence orbitals up to 21 eV should transform as 3 X al, 1 X a,, 2 X bl, and 3 X b2. DISCUSSION
Assignment
of the H,C(CN),
PE spectrum
Given the density of states up to 15 eV, and the Koopmans’ problems associated with u CN orbitals (nN) this is not a trivial matter. It is clear that the degeneracy of a linear dicyano species is removed in a Czv molecule, and the through space and through bond interactions give the ordering b 1, b2, a2 and a1 for the four ncN orbitals [ 1, 21. What is not clear is the precise location
IONlZATlON
POTENTIAL
(eV)
Fig. 2. Ne I spectra (corrected for the doublet intensity) of H,C(CN),, Br,C(CN), in the 11-15 eV region.
Cl,C(CN),
and
of the remaining two nN orbitals (ai and b,), and it is here unfortunately that calculations that do not include correlation are particularly poor. These six orbit& are to be accommodated over the first six IPs given in Table 1 which span only 1.34 eV. The first band (12.72 eV) has extensive vibrational structure, and overlaps the second (Fig. 1) at 13.14 eV. Two distinct peaks are evident at 13.42 and 13.59 eV and two overlapping bands lie at 13.91 and 14.06 eV. Three remaining bands occur at 17.51, 18.15 and 19.34 eV. Our assignment, for reasons given below, differs in a smah but significant Way from the ircN, ncN, n&, n;, 7rcN,ncN ordering of SIX&& and Bock [2] in the 12.5-14.5 eV region; we prefer moving the two nN orbitals to slightly higher IP (fromamean of 13.51 eV to a mean of 13.75 eV), to give the order TCN, TCN, TICN~ 4% nii, and ,rcN. The placement of the nN orbitals at 13.59 and 13.91 eV is based on the sharp, strong appearance of these bands in the He I spectrum and the changes associated with going to Ne I radiation where, due to the 2s contribution in these orbitals, their intensities drop relative to the other bands. Conversely,
345 TABLE
1
Experimental IP (eV)” 12.72
13.14 13.42 13.59 13.91 14.06 17.51
and calculated
IPs for H,C(CN),
Vibrational Structure (cm-‘)a
Present assignmentb
970 2430 2850
520 670 1350
18.1 19.3
IP (eV)
HAM/B=
MNDOd
MS Xoie
2b, (~cN)
11.90
12.97
13.74
4b, 1% 5a, 3b,
4% (nc?J) lb, (nq)
12.39 12.54 12.55 13.12 13.13 16.19
13.50 13.82 13.47 14.70 15.03 16.59
14.30 14.45 14.37 16.98 16.96 16.25
3a, (0) 2b, (0)
17.18 17.42
18.28 20.79
17.50 19.38
(~cN) (TCN) (nk) (%)
aIPs, kO.05 eV;vibrational structure *60 cmwave geometry, ref. 6. dOptimised geometry,
TABLE
Calculated
’ . bValence
orbital
see structure
labelling;
in text, eRef.
see text.
CMicro-
12.
2
Experimental
and calculated
IPs for Cl,C(CN),
(eV)”
Present assignmentb
Calculated IP (eV)
12.6 12.78 12.78 13.26 13.59 13.59 13.8 14.20 14.20 14.6 17.26 17.79
4b, 2a, 56, 7% 3b, 4b, la, 6~1 3h 5a, 2b, 4%
13.37 13.70 13.64 13.75 13.93 14.21 14.72 14.63 15.67 15.84 18.15 19.45
IP
aIPs, to.05 COptimized
eV, except for IPs 1, 7 and 10, iO.1 geometry; see structure in text.
eV. bValence
orbital
labelling;
(MNDO)C
see text.
He II results [l] show these same bands to increase in relative intensity. In addition, since the 3al orbital is now unambiguously assigned to 18.15 eV (see below and ref. l), the 4a1 (n+N)orbital is now subjected to less through bond interaction.
346 TABLE
3
Experimental
11.48 11.87 12.01 12.40 12.94 13.2 13.4 13.70 13.87 14.11 16.42 17.03
and calculated
IPs for Br,C(CN), Present assignmentb
Calculated IP (eV)
4b, 2% 5b, 7a, 3b, 4b, la, 6% 3b, 5a, 2b, 4a,
12.39 12.85 12.86 13.03 13.47 14.02 14.43 14.21 15.41 15.63 17.35 18.82
aIPs, ~0.05 eV, except for IPs 6 and 7, +O.l eV. bValence mized geometry; see structure in text.
orbital
labelling;
(MNDO)C
see text.
COpti-
It has been found that nitrile proton affinities can be linearly related to nN IPs [14], thereby providing a means of locating these peaks. In the case of H,C(CN), and using the previous assignment 121, the points for n& and n& were found to lie some 6-10 kcal mol-’ from the predicted line. Our new assignment places the n, orbital right on the line, and the through bond destabilisation of the nk orbital is reduced. This gives a homolytic bond dissociation oD(B’-H) of 177.7 kcal mol-‘, closer to the expected value of 184.1 kcal mol-l. We also note that by moving the nN orbitals to slightly higher IPs our assignment now places the unique a2 orbital at 13.42 eV, closer to its position in the valence isoelectronic S(CN), molecule (13.59 eV) [15]. It has been established that calculations employing Koopmans’ theorem to assign measured IPs to calculated orbital energies break down for cyanocontaining molecules [l, 151, the problem being attributed to neglect of correlation. However the HAM/3 and MS X, methods, because they use the transition state formalism, get around the problem of correlation and therefore may fare better for these types of molecule. Table 1 shows the results of the HAM/3 and MS X, [12] calculations together with values obtained using MNDO. In all cases the calculations show a clear distinction between the 7rcN orbitals (lOWeSt IP) and the nN orbitals (essentially degenerate in HAM/3 and MS X,). These computational results should therefore be taken cum guano salis although there are some general features that we can mention. The HOMO is b, (rcN) in all cases, separated from the three remaining It should be noted that the calculations place closely spaced TcN orbit&. the cl2 and a, TcN orbitals very close in energy (indeed switched in MNDO). The parameterised MO model [l, 21 indicates, however, that the through
347
bond interaction of the a1 orbit& is insufficient to effect this switch. Then come the two nN orbit& (although in the MS X, case there is an intervening bl level), the remaining three orbitals up to 21 eV having the ordering bl, a1 and bz. The specific region of interest, however, corresponds to the four 7rcNand two nN orbitals. We shall not use the MNDO results since they are based on the Koopmans’ assumption although, in general, MNDO has provided good qualitative and quantitative agreement with experimental IPs for a wide range of molecules. In the MS X, case the nN orbitals are substantially stabilised (to 17 eV), a feature ascribed to neglect of correlation [12], but also, perhaps, due to an inability to choose the correct sphere size for multiple bonds. The HAM/3 results which have a demonstrated performance for a wide variety of molecules [16], fare considerably better; the spread of the six orbitals parallels that for the experimental results although the centre of gravity of the group is ca. 1 eV too low. The ordering is close to the one we have developed above on the basis of the spectra and other information, except that the nN orbitals are not interposed between the a2 and a1 ncN levels. Taken overall, we feel that this result is also some additional justification for placing the nN orbitals to slightly higher energy than the original assignment [2]. As we shall see below this ordering can then provide some self-consistency with the results for the dihalodicyano molecules. Assignment
of the BT-&(CN)~ PE spectrum
This molecule is tackled first as there is a clear separation between orbitals due to the BrZC fragment (11-12.5 eV; al, uz, bl, 13,)and the C(CN)* moiety (13-14.5 eV). The assignments for BrzCH2 have been discussed before [S] ; we simply note that in order to retain the 7rcNorbitals as b1 and a2 symmetry, the b, and bz orbitals of BrzCH2 [8] should now be classified as bz and bl. Because of the high electronegativity of the CN groups the Br “lone pair” orbitals are stabilised by some 1 eV, and the IP pattern is somewhat different from that in BrzCH2 [ 81. The overall ordering will thus be a balance of inductive effects and orbital interactions which may partially cancel. The band shapes and He I/Ne I intensities can, however, provide clues to a plausible assignment. Thus the broad first IP at 11.48 eV is retained as the bl orbital, with one of the sharp bands at about 11.9 eV belonging to the a2 orbital. The first (11.87 eV) is preferred since the intensity is enhanced in the Ne I spectrum (Fig. 2). The b2 orbital can then be assigned to the band at 12.01 eV, with the band at 12.40 eV belonging to the a, orbital. The latter decreases in intensity with Ne I radiation, indicative of some strong orbital interaction. This assignment of the nar region gives a very similar ordering to that in Br2CClZ [17] (except for the very closely spaced a2 and bz orbitals) and follows the MNDO calculations (Table 3). The ncN/nNregion has been modified, not so much with regard to position, but rather with respect to the relative intensities of bands. In particular those
348
IPs which arose from orbitals strongly localised on N, i.e. rzh and nk, no longer appear as sharp distinct peaks since there is now some X p character in these MOs. This appears also to be the case for the chlorinated methyl cyanides where addition of chlorine atoms delocalises the N lone pair [ 181. Since most IPs have barely shifted in position, and the calculations show the extent of mixing of the rrCN and nsr orbitals of the same symmetry to be relatively small, we take the ordering of orbitals to be the same as in H,C(CN),. Certainly the band in Br,C(CN), at 12.94 eV corresponds in both shape and position to the b1 orbital observed in H,C(CN), at 12.72 eV. The next IP at 13.2 eV assigned to a bz orbital has barely moved from the position in H,C(CN), (13.14 eV); this is not surprising since there is minimal mixing with the corresponding nBr orbital. The next three bands at 13.4,13.70 and 13.87 eV are assigned in the order 77CN(aZ), n’,(al) and n&(b,) although this ordering must be considered tentative given the lack of discernible structure. The final band in this group at 14.11 eV is then the n&a,) orbital, which again is broad and featureless. The calculations (Table 3) for this group of tightly spaced orbitals show the nN orbitals to be well separated, which is again a reflection of the inability to handle localised N type orbit&. The 16.42 and 17.03 eV bands are assigned to bl(n & and al(a) orbitals respectively. A b2 orbital is also expected in the high IP region and no obvious band exists even though our spectrometer does not discriminate against low energy electrons. It may well occur beyond the energy of the He I light source; its predicted position (MNDO) being 22.18 eV. Assignment
of the Cl,C(CN)2
PE spectrum
This spectrum (Figs. 1 and 2) shows that the ncl and 7rcN/nN regions are much closer in energy, resulting in considerable orbital mixing and severe overlap. With ten IPs over a 2.5 eV region it is difficult to make specific assignments, and given that only four main bands are in evidence (with some suggestions of shoulders) it is clear that even a parameterised model will fail in these circumstances. A proposed assignment is shown in the correlation diagram of Fig. 3. The first three ncl orbitals bl, az, bl, are placed in the intense band centred at 12.7 eV; this is reasonable given the intensity increase of this band on going to Ne I radiation. The a1 orbital is then placed at 13.26 eV, leaving the six ncN/nN orbitals to be accommodated in the range 13.5-14.5 eV. On intensity arguments the b, and b2 rcN orbitals are superimposed at 13.59 eV, the weak ncN az orbital is observed as a shoulder at 13.8 eV and the remaining three bands ul(rCN), n& and n, occur in the broad intense band at 14.2 eV. These assignments are by no means firm but, based on intensity and intensity change arguments and a correlation with the Br,C(CN), spectrum, appear reasonable.
349
B&N)2
Br2CH2
“P?h
C’2W2
Cl*CH*
“I -\
b2-\
13
\%
1
Fig. 3. Correlation
of the experimental
IPs of Br,CH,,
Br,C(CN),,
H,C(CN),,
Cl,C(CN),
and Cl,CH,.
CONCLUSION
The previously unreported uv photoelectron spectra of dichloro- and dibromomalononitrile have been obtained and analysed on the basis of comparisons with the X2C and C(CN)* fragments and intensity changes between the He I and Ne I light sources. Given the great number of closely spaced IPs and the paucity of resolvable vibrational structure, the assignments must be regarded as tentative. That for Br2C(CN)z involves the least perturbation between the Br and CN moieties and is probably reasonable. In the Cl,C(CN), case severe overlap precludes specific assignments although some generalisations can be made. The HAM/3 calculations (for H,C(CN),) and the MNDO calculations for H,C(CN)2 and X,C(CN), indicate severe Koopmans’ problems for the strongly localised N MOs. They do, however, provide some guide to the ordering and mixing of the other orbitals. ACKNOWLEDGMENT
NPCW gratefully acknowledges the financial and Engineering Research Council of Canada.
support of the Natural Sciences
REFERENCES 1 H. Stafast and H. Bock, in S. Patai and Z. Rappoport (Eds.), The Chemistry tional Groups, Suppl. C, Part 1, Wiley, New York, 1983. 2 H. Stafast and H. Bock, Z. Naturforsch., Teil B 28, (1973) 746.
of Func-
350 3 F. Halverson and R. J. Francel, J. Chem. Phys., 17 (1949) 694. 4 T. Fujiyama and T. Shimanouchi, Spectrochim. Acta, 20 (1949) 829. 5 L. Van Haverbeke, H. 0. Desseyn and B. J. Van Der Veken, Bull. Sot. Chim. Belg., 82 (1973) 133. 6 E. Hirota and Y. Morino, Bull. Chem. Sot. Jpn., 33 (1960) 158, 705. 7 K. Obatake and S. Tanisaki, Phys. Lett. A, 44 (1973) 341. 8 R. N. Dixon, J. N. Murrell and B. Narayan, Mol. Phys., 20 (1971) 611. 9 T. Hata, Bull. Chem. Sot. Jpn., 37 (1964) 547. 10 J. J. P. Stewart, MOPAC, Q.C.P.E. No. 455. 11 L. Asbrink, C. Fridh and E. Lindholm, Chem. Phys. Lett., 52 (1977) 69. 12 J. Weber, N. Thalmann and E. Haselbach, Chem. Phys. Lett., 57 (1978) 230. 13 J. Mirek and A. Buda, Z. Naturforsch., Teil A, 38 (1983) 774. 14 R. H. Staley, J. E. Kleckner and J. L. Beauchamp, J. Am. Chem. Sot., 98 (1976) 2081. 15 P. Rosmus, H. Stafast and H. Bock, Chem. Phys. Lett., 34 (1975) 275. 16 D. C. Frost, W. M. Lau, C. A. McDowell and N. P. C. Westwood, J. Mol. Struct. (Theothem), 90 (1982) 283. 17 J. C. Bunzli, D. C. Frost, F. G. Herring and C. A. McDowell, J. Electron Spectrosc., Relat. Phenom., 9 (1976) 289. 18 R. F. Lake and H. Thompson, Proc. R. Sot. London, Ser. A, 317 (1970) 187.
Science
Publishers
B.V.,
162 (1987)
341-350
Amsterdam
-
He I AND Ne I PHOTOELECTRON AND DIBROMOMALONONITRILE
C. B. MACDONALD,
D. S. WADDELL
Printed
in The Netherlands
SPECTRA OF DICHLORO-
and N. P. C. WESTWOOD*
Guelph-Waterloo Centre for Graduate Work in Chemistry, Guelph, Ontario Nl G 2 Wl (Canada) (Received
12 June
University of Guelph,
1987)
ABSTRACT He I and Ne I photoelectron spectra are reported for the densely populated valence regions of gaseous malononitrile, H,C(CN),, dichloromalononitrile, Cl,C(CN),, and dibromomalononitrile, Br,C(CN),. A minor reassessment of the H,C(CN), assignments is extended to permit plausible assignments for the previously unreported dihalodicyano species. Semiempirical calculations, HAM/3 and MNDO for H,C(CN),, and MNDO for the X,C(CN), molecules, are shown to be of limited value for the location of strongly localised nitrogen orbitals.
INTRODUCTION
The photoelectron (PE) spectra of many CN containing molecules are known, and are often complex due to the overlapping rcN and ncN orbitals in the 12-l 5 eV range [l]. In addition, even ab initio SCF calculations do not adequately represent the measured ionisation potentials (IPs) of such compounds due to severe Koopmans’ problems. Of relevance to this work is the parent Czv molecule, dicyanomethane (malononitrile) the PE spectrum of which has been recorded and assigned using a simple MO model based on through space and through bond interactions [2]. In this work we evaluate both the He I and Ne I PE spectra for malononitrile and the efficacy (or otherwise) of semiempirical (MNDO) and HAM/3 calculations. We then extend these results to the He I and Ne I PE spectra of the dihalo species X,C(CN),. Here, ionisation due to the halogen (X) lone pairs is expected to complicate the issue with a grand total of 10 (X + CN) valence IPs up to 15 eV. The overall Czv structures for these molecules are substantiated by the IR and Raman spectra of H,C(CN)* [3, 41, ClzC(CN)2 [5] and Br,C(CN), [5], in addition to the microwave spectrum [6] and crystallographic [7] studies
*Author
to whom
0022-2860/87/$03.50
correspondence 0 1987
should
be addressed.
Elsevier
Science
Publishers
B.V.
342
of H,C(CN),. This facilitates a combination of the assignments for dicyanomethane with the known assignments for the Czv dihalomethanes [S] for evaluating the spectra of the dihalodicyano molecules. EXPERIMENTAL
H,C(CN), (Eastman) was purified by recrystallisation from ether, and dried in vacua before use. Cl,C(CN), (Aldrich) was used without further purification. Br,C(CN), was prepared by addition of Brz to aqueous H,C(CN), as previously described [ 91. Photoelectron spectra were obtained on a specially constructed spectrometer with a resolution (under sample operating conditions) of 30 meV. The He I PE spectra were used to obtain accurate IPs, calibration being effected with the known IPs of Ar, N2 and CHJI. The Ne I light source is an approximate 5:l doublet at 16.85 and 16.67 eV. Thus, in order to obtain more meaningful relative intensities, ionisation due to the weaker line was digitally subtracted; this procedure was monitored using the band corresponding to the “Z:‘g state of N: in order to ensure fidelity. MNDO calculations were performed using the MOPAC package [lo]. HAM/3 calculations [ll] were also performed for H,C(CN), to assist with the assignments. The calculations were implemented on a VAX 11/750 computer. RESULTS
The He I PE spectra of H,C(CN),, Cl,C(CN), and Br,C(CN), are shown in Fig. 1, and the corrected Ne I spectra up to 15 eV are shown in Fig. 2. IPs and vibrational frequencies for H,C(CN), are listed in Table 1 along with the results of the present MNDO and HAM/3 calculations, and some multiple scattering MS X, calculations [12]. The third column shows the current assignments, which are discussed below. The IP values are in good agreement with those reported earlier [2]. Tables 2 and 3 show the measured IPs and proposed assignments for the dichloro- and dibromo- derivatives, respectively, together with the MNDO calculated values. The calculated (MNDO) structures for H,C(CN),, Cl,C(CN), and Br,C(CN),
N
N
N
That for H,C(CN), is in reasonable agreement with results obtained from microwave spectroscopy [ 61, including slight non-linearity in the CCN angle.
343
t
j\bL h----I 11
_._12
1
IONIZATION
POTENTIAL
Fig. 1. He I photoelectron
v
17
15
13
(eV)
spectra of H,C(CN),,
Cl,C(CN), and Br,C(CN),.
The results for the X,C(CN), molecules should provide a good starting point for structural investigations. In addition to those results shown in Table 1, some earlier MNDO results are available [13] for H,C(CN), but these are not included since, although some values correlate with our results, there are severe discrepancies/typographical errors in the numbers and orbital assignments, e.g. the results [13] show 4 X al, 4 X bl, and 1 X bz orbitals, whereas the nine available valence orbitals up to 21 eV should transform as 3 X al, 1 X a,, 2 X bl, and 3 X b2. DISCUSSION
Assignment
of the H,C(CN),
PE spectrum
Given the density of states up to 15 eV, and the Koopmans’ problems associated with u CN orbitals (nN) this is not a trivial matter. It is clear that the degeneracy of a linear dicyano species is removed in a Czv molecule, and the through space and through bond interactions give the ordering b 1, b2, a2 and a1 for the four ncN orbitals [ 1, 21. What is not clear is the precise location
IONlZATlON
POTENTIAL
(eV)
Fig. 2. Ne I spectra (corrected for the doublet intensity) of H,C(CN),, Br,C(CN), in the 11-15 eV region.
Cl,C(CN),
and
of the remaining two nN orbitals (ai and b,), and it is here unfortunately that calculations that do not include correlation are particularly poor. These six orbit& are to be accommodated over the first six IPs given in Table 1 which span only 1.34 eV. The first band (12.72 eV) has extensive vibrational structure, and overlaps the second (Fig. 1) at 13.14 eV. Two distinct peaks are evident at 13.42 and 13.59 eV and two overlapping bands lie at 13.91 and 14.06 eV. Three remaining bands occur at 17.51, 18.15 and 19.34 eV. Our assignment, for reasons given below, differs in a smah but significant Way from the ircN, ncN, n&, n;, 7rcN,ncN ordering of SIX&& and Bock [2] in the 12.5-14.5 eV region; we prefer moving the two nN orbitals to slightly higher IP (fromamean of 13.51 eV to a mean of 13.75 eV), to give the order TCN, TCN, TICN~ 4% nii, and ,rcN. The placement of the nN orbitals at 13.59 and 13.91 eV is based on the sharp, strong appearance of these bands in the He I spectrum and the changes associated with going to Ne I radiation where, due to the 2s contribution in these orbitals, their intensities drop relative to the other bands. Conversely,
345 TABLE
1
Experimental IP (eV)” 12.72
13.14 13.42 13.59 13.91 14.06 17.51
and calculated
IPs for H,C(CN),
Vibrational Structure (cm-‘)a
Present assignmentb
970 2430 2850
520 670 1350
18.1 19.3
IP (eV)
HAM/B=
MNDOd
MS Xoie
2b, (~cN)
11.90
12.97
13.74
4b, 1% 5a, 3b,
4% (nc?J) lb, (nq)
12.39 12.54 12.55 13.12 13.13 16.19
13.50 13.82 13.47 14.70 15.03 16.59
14.30 14.45 14.37 16.98 16.96 16.25
3a, (0) 2b, (0)
17.18 17.42
18.28 20.79
17.50 19.38
(~cN) (TCN) (nk) (%)
aIPs, kO.05 eV;vibrational structure *60 cmwave geometry, ref. 6. dOptimised geometry,
TABLE
Calculated
’ . bValence
orbital
see structure
labelling;
in text, eRef.
see text.
CMicro-
12.
2
Experimental
and calculated
IPs for Cl,C(CN),
(eV)”
Present assignmentb
Calculated IP (eV)
12.6 12.78 12.78 13.26 13.59 13.59 13.8 14.20 14.20 14.6 17.26 17.79
4b, 2a, 56, 7% 3b, 4b, la, 6~1 3h 5a, 2b, 4%
13.37 13.70 13.64 13.75 13.93 14.21 14.72 14.63 15.67 15.84 18.15 19.45
IP
aIPs, to.05 COptimized
eV, except for IPs 1, 7 and 10, iO.1 geometry; see structure in text.
eV. bValence
orbital
labelling;
(MNDO)C
see text.
He II results [l] show these same bands to increase in relative intensity. In addition, since the 3al orbital is now unambiguously assigned to 18.15 eV (see below and ref. l), the 4a1 (n+N)orbital is now subjected to less through bond interaction.
346 TABLE
3
Experimental
11.48 11.87 12.01 12.40 12.94 13.2 13.4 13.70 13.87 14.11 16.42 17.03
and calculated
IPs for Br,C(CN), Present assignmentb
Calculated IP (eV)
4b, 2% 5b, 7a, 3b, 4b, la, 6% 3b, 5a, 2b, 4a,
12.39 12.85 12.86 13.03 13.47 14.02 14.43 14.21 15.41 15.63 17.35 18.82
aIPs, ~0.05 eV, except for IPs 6 and 7, +O.l eV. bValence mized geometry; see structure in text.
orbital
labelling;
(MNDO)C
see text.
COpti-
It has been found that nitrile proton affinities can be linearly related to nN IPs [14], thereby providing a means of locating these peaks. In the case of H,C(CN), and using the previous assignment 121, the points for n& and n& were found to lie some 6-10 kcal mol-’ from the predicted line. Our new assignment places the n, orbital right on the line, and the through bond destabilisation of the nk orbital is reduced. This gives a homolytic bond dissociation oD(B’-H) of 177.7 kcal mol-‘, closer to the expected value of 184.1 kcal mol-l. We also note that by moving the nN orbitals to slightly higher IPs our assignment now places the unique a2 orbital at 13.42 eV, closer to its position in the valence isoelectronic S(CN), molecule (13.59 eV) [15]. It has been established that calculations employing Koopmans’ theorem to assign measured IPs to calculated orbital energies break down for cyanocontaining molecules [l, 151, the problem being attributed to neglect of correlation. However the HAM/3 and MS X, methods, because they use the transition state formalism, get around the problem of correlation and therefore may fare better for these types of molecule. Table 1 shows the results of the HAM/3 and MS X, [12] calculations together with values obtained using MNDO. In all cases the calculations show a clear distinction between the 7rcN orbitals (lOWeSt IP) and the nN orbitals (essentially degenerate in HAM/3 and MS X,). These computational results should therefore be taken cum guano salis although there are some general features that we can mention. The HOMO is b, (rcN) in all cases, separated from the three remaining It should be noted that the calculations place closely spaced TcN orbit&. the cl2 and a, TcN orbitals very close in energy (indeed switched in MNDO). The parameterised MO model [l, 21 indicates, however, that the through
347
bond interaction of the a1 orbit& is insufficient to effect this switch. Then come the two nN orbit& (although in the MS X, case there is an intervening bl level), the remaining three orbitals up to 21 eV having the ordering bl, a1 and bz. The specific region of interest, however, corresponds to the four 7rcNand two nN orbitals. We shall not use the MNDO results since they are based on the Koopmans’ assumption although, in general, MNDO has provided good qualitative and quantitative agreement with experimental IPs for a wide range of molecules. In the MS X, case the nN orbitals are substantially stabilised (to 17 eV), a feature ascribed to neglect of correlation [12], but also, perhaps, due to an inability to choose the correct sphere size for multiple bonds. The HAM/3 results which have a demonstrated performance for a wide variety of molecules [16], fare considerably better; the spread of the six orbitals parallels that for the experimental results although the centre of gravity of the group is ca. 1 eV too low. The ordering is close to the one we have developed above on the basis of the spectra and other information, except that the nN orbitals are not interposed between the a2 and a1 ncN levels. Taken overall, we feel that this result is also some additional justification for placing the nN orbitals to slightly higher energy than the original assignment [2]. As we shall see below this ordering can then provide some self-consistency with the results for the dihalodicyano molecules. Assignment
of the BT-&(CN)~ PE spectrum
This molecule is tackled first as there is a clear separation between orbitals due to the BrZC fragment (11-12.5 eV; al, uz, bl, 13,)and the C(CN)* moiety (13-14.5 eV). The assignments for BrzCH2 have been discussed before [S] ; we simply note that in order to retain the 7rcNorbitals as b1 and a2 symmetry, the b, and bz orbitals of BrzCH2 [8] should now be classified as bz and bl. Because of the high electronegativity of the CN groups the Br “lone pair” orbitals are stabilised by some 1 eV, and the IP pattern is somewhat different from that in BrzCH2 [ 81. The overall ordering will thus be a balance of inductive effects and orbital interactions which may partially cancel. The band shapes and He I/Ne I intensities can, however, provide clues to a plausible assignment. Thus the broad first IP at 11.48 eV is retained as the bl orbital, with one of the sharp bands at about 11.9 eV belonging to the a2 orbital. The first (11.87 eV) is preferred since the intensity is enhanced in the Ne I spectrum (Fig. 2). The b2 orbital can then be assigned to the band at 12.01 eV, with the band at 12.40 eV belonging to the a, orbital. The latter decreases in intensity with Ne I radiation, indicative of some strong orbital interaction. This assignment of the nar region gives a very similar ordering to that in Br2CClZ [17] (except for the very closely spaced a2 and bz orbitals) and follows the MNDO calculations (Table 3). The ncN/nNregion has been modified, not so much with regard to position, but rather with respect to the relative intensities of bands. In particular those
348
IPs which arose from orbitals strongly localised on N, i.e. rzh and nk, no longer appear as sharp distinct peaks since there is now some X p character in these MOs. This appears also to be the case for the chlorinated methyl cyanides where addition of chlorine atoms delocalises the N lone pair [ 181. Since most IPs have barely shifted in position, and the calculations show the extent of mixing of the rrCN and nsr orbitals of the same symmetry to be relatively small, we take the ordering of orbitals to be the same as in H,C(CN),. Certainly the band in Br,C(CN), at 12.94 eV corresponds in both shape and position to the b1 orbital observed in H,C(CN), at 12.72 eV. The next IP at 13.2 eV assigned to a bz orbital has barely moved from the position in H,C(CN), (13.14 eV); this is not surprising since there is minimal mixing with the corresponding nBr orbital. The next three bands at 13.4,13.70 and 13.87 eV are assigned in the order 77CN(aZ), n’,(al) and n&(b,) although this ordering must be considered tentative given the lack of discernible structure. The final band in this group at 14.11 eV is then the n&a,) orbital, which again is broad and featureless. The calculations (Table 3) for this group of tightly spaced orbitals show the nN orbitals to be well separated, which is again a reflection of the inability to handle localised N type orbit&. The 16.42 and 17.03 eV bands are assigned to bl(n & and al(a) orbitals respectively. A b2 orbital is also expected in the high IP region and no obvious band exists even though our spectrometer does not discriminate against low energy electrons. It may well occur beyond the energy of the He I light source; its predicted position (MNDO) being 22.18 eV. Assignment
of the Cl,C(CN)2
PE spectrum
This spectrum (Figs. 1 and 2) shows that the ncl and 7rcN/nN regions are much closer in energy, resulting in considerable orbital mixing and severe overlap. With ten IPs over a 2.5 eV region it is difficult to make specific assignments, and given that only four main bands are in evidence (with some suggestions of shoulders) it is clear that even a parameterised model will fail in these circumstances. A proposed assignment is shown in the correlation diagram of Fig. 3. The first three ncl orbitals bl, az, bl, are placed in the intense band centred at 12.7 eV; this is reasonable given the intensity increase of this band on going to Ne I radiation. The a1 orbital is then placed at 13.26 eV, leaving the six ncN/nN orbitals to be accommodated in the range 13.5-14.5 eV. On intensity arguments the b, and b2 rcN orbitals are superimposed at 13.59 eV, the weak ncN az orbital is observed as a shoulder at 13.8 eV and the remaining three bands ul(rCN), n& and n, occur in the broad intense band at 14.2 eV. These assignments are by no means firm but, based on intensity and intensity change arguments and a correlation with the Br,C(CN), spectrum, appear reasonable.
349
B&N)2
Br2CH2
“P?h
C’2W2
Cl*CH*
“I -\
b2-\
13
\%
1
Fig. 3. Correlation
of the experimental
IPs of Br,CH,,
Br,C(CN),,
H,C(CN),,
Cl,C(CN),
and Cl,CH,.
CONCLUSION
The previously unreported uv photoelectron spectra of dichloro- and dibromomalononitrile have been obtained and analysed on the basis of comparisons with the X2C and C(CN)* fragments and intensity changes between the He I and Ne I light sources. Given the great number of closely spaced IPs and the paucity of resolvable vibrational structure, the assignments must be regarded as tentative. That for Br2C(CN)z involves the least perturbation between the Br and CN moieties and is probably reasonable. In the Cl,C(CN), case severe overlap precludes specific assignments although some generalisations can be made. The HAM/3 calculations (for H,C(CN),) and the MNDO calculations for H,C(CN)2 and X,C(CN), indicate severe Koopmans’ problems for the strongly localised N MOs. They do, however, provide some guide to the ordering and mixing of the other orbitals. ACKNOWLEDGMENT
NPCW gratefully acknowledges the financial and Engineering Research Council of Canada.
support of the Natural Sciences
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