2015
J . Phys. Chem. 1986,90, 2015-2019 The simulation of the N I S spectra has been used as a visual proof of the assignment; it does not prove the uniqueness of the solution but many other possibilities can be rejected on the basis of the fit between simulated and experimental spectra.
their careful investigation of the infrared spectra, and 0. Martin-Borret for her participation in the synthesis.
Acknowledgment. We are very much indebted to A. Chosson and C. Corne (Laboratoire de Spectroscopie Optique, Universiti de Savoie, Chambiry, France) for their help in recording the Raman spectra. We thank Dr. J. C. Lassggues and Dr. B. Desbat (Laboratoire de Spectroscopie Infrarouge, Bordeaux, France) for
Supplementary Material Available: Observed and calculated vibrational modes for the dimethylnorbornanes along with their potential energy distribution, Table i for 4a and 4b,Table ii for 5a and Sb,and Table iii for 6a and 6b. (12 pages). Ordering information is given on any current masthead page.
Registry No. 4a, 37602-64-5; 4b, 100928-82-3; 5a, 20558-16-1; 5b, 100992-52-7; 6a, 28626-68-8; 6b, 100928-83-4.
Penning Ionization Electron Spectroscopy of Molecules Containing the C=O Aldehydes and Carboxylic Acids
Group.
Koichi Ohno,* Shuro Takano, and Kazuhiko Mase Department of Chemistry, The College of Arts and Sciences, The University of Tokyo, Komaba, Meguro- ku, Tokyo 153, Japan (Received: May 28, 1985; In Final Form: December 30, 1985)
He*(2,S) Penning ionization electron spectra (PIES) and He I photoelectron spectra of COz, HCHO, CHSCHO,CH,CH,CHO, HCOOH, CH,COOH, and CH3CH2COOH were measured with a transmission-corrected electron spectrometer. The relative band intensities in PIES were analyzed on the basis of ab initio MO calculations. It is found that an isolobal analogy holds for the relative reactivity of target molecular orbitals upon electrophilic attacks by metastable helium atoms.
-
Introduction In Penning ionization of molecules M (M + A* M+ A + e-), metastable rare gas atoms A* attack occupied orbitals of M from which an electron is transferred to the inner-shell orbital of A* in association with an electron emission from the outer shell of A* into a continuum state.' Kinetic energy analyses of ejected electrons provide Penning ionization electron spectra (PIES), which are similar in many respects to UV photoelectron spectra (UPS).2 Recent studies, however, have revealed that occupied molecular orbitals whose electron distributions are extending outside give strong bands in PIES.3,4 This characteristic of PIES has been understood in terms of the electron-exchange mechanism in which exterior electron distributions of molecular orbitals are selectively probed by incoming metastable atoms. Such a nature of PIES provides a valuable means for the assignments of UV photoelectron spectra,'-* the study of spatial distributions of molecular orbital^,^,^ and the study of solid s u r f a ~ e s . ~ - l ~ Since the relative reactivity of orbitals can be probed by utilizing
+
(1) Hotop, H.; Niehaus, A. Z . Phys. 1969, 228, 68. (2) Cermlk, V. J . Chem. Phys. 1966, 44, 3781. (3) Ohno, K.; Mutoh, H.; Harada, Y. J . Am. Chem. SOC.1983,105,4555. (4) Ohno, K.; Matsumoto, S.; Harada, Y. J . Chem. Phys. 1984,81,4447. (5) Munakata, T.; Kuchitsu, K.; Harada, Y. Chem. Phys. Lett. 1979,64, 409. (6) Veszprtmi, T.;Harada, Y.; Ohno, K.; Mutoh, H. J. Organomet. Chem. 1983, 224, 115; 1983, 252, 121; 1984, 266, 9. (7) Munakata, T . ;Ohno, K.; Harada, Y.; Kuchitsu, K. Chem. Phys. Lett. 1981, 83, 243. (8) Veszpremi, T.; Bihltsi, L.; Harada, Y.; Ohno, K.; Mutoh, H. J . Organomet. Chem. 1985, 280, 39. (9) Munakata, T.; Ohno, K.; Harada, Y. J . Chem. Phys. 1980,72, 2880. (10) Kubota, H.; Munakata, T.; Hirooka, T.; Kuchitsu, K.; Harada, Y. Chem. Phys. Lett. 1980, 74, 409. (11) Ohno, K.; Mutoh, H.; Harada, Y. Surf. Sci. 1982, 115, L128. (12) Harada, Y.; Ozaki, H.; Ohno, K. Phys. Rev. Lett. 1984, 52, 2269. (13) Harada, Y.; Ozaki, H.; Ohno, K.; Kajiwara, T. Surf.Sci. 1984, 147, 356.
0022-3654/86/2090-201 S O 1 S O / O
electrophilic attacks of metastable rare gas atoms, applications of this PIES technique to various organic molecules are highly interesting. In earlier work, a characteristic band with an extremely large intensity was observed for molecules containing the C N group.I4-l6 The relative activity of orbitals localized on functional groups (-NH2, -OH, -SH,-C1, -I) was found to depend on the electronegativity of the heteroatoms." Steric shielding effects of bulky groups protecting the target orbitals from attacks of metastable atoms were also found for some substituted anilines,l8 nitriles,16 and somu organometallic c o m p l e x e ~ . ' ~ ~ ~ ~ In the present study, PIES and UPS of COz, HCHO, CH3CHO, CH3CH2CH0,HCOOH, CH,COOH, and CH3CH2COOH were measured with a transmission-corrected electron spectrometer. A band with strong intensity was found for each molecule in PIES and assigned to a (r orbital having a characteristic electron distribution on the C=O group. Band intensities in PIES and spatial distributions of molecular orbitals are discussed on the basis of ab initio MO calculations.
Experimental Section He*(23S) PIES and H e I UPS were measured by a transmission-corrected electron spectrometer.3*20The helium metastable atoms, 23S (19.82 eV) and 2 ' s (20.62 eV), were produced by impact of 60-eV electrons with the beams of helium atoms col(14) Cermlk, V.; Yencha, A. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 339. (15) Yee, D. S. C.; Stewart, W. B.; McDowell, C. A,; Brion, C. E. J . Electron Spectrosc. Relar. Phenom. 1975, 377, I. (16) Ohno, K.; Matsumoto, S.; Imai, K.; Harada, Y. J . Phys. Chem. 1984, 88, 206. (17) Ohno, K.; Imai, K.; Matsumoto, S.; Harada, Y. J. Phys. Chem. 1983, 87, 4346. (18) Ohno, K.; Fujisawa, S.; Mutoh, H.; Harada, Y. J . Phys. Chem. 1982, 86, 440. (19) Munakata, T.;Harada, Y.; Ohno, K.; Kuchitsu, K. Chem. Phys. Lett. 1981, 84, 6. (20) Harada, Y.; Ohno, K.; Mutoh, H. J . Chem. Phys. 1983, 79, 3251.
0 1986 American Chemical Society
2016
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986
Ohno et al.
limated through a fused glass-capillary array. A water-cooled helium discharge lamp was used as a quench lamp to eliminate 2IS atoms from the metastable beams. In the resultant metastable beams, the 2% component is sup ressed to less than a few percent. The He I resonance line (584 , 21.22 eV) was produced by a dc discharge in pure helium gas. A gaseous sample was introduced into the ionization chamber through a Whitey needle valve so as torr in the to control the sample pressure at about (1-2) X main chamber. The electron spectra were obtained at an ejection angle of 90' with respect to the metastable atom beam or the photon beam by means of a hemispherical analyzer with a computer-controlled retarding system. Signals were accumulated on a signal-averaging system with a microcomputer. The energy dependence of the transmission of the electron spectrometer was determined by a detailed study of the UPS of 02,CO, C 0 2 , N2, and some hydrocarbons. Corrections of the stored spectral data by the transmission curves were made on the microcomputer.
w
Calculations Ab initio SCF MO calculations with 4-31G basis functions were performed by a library program at the computer center of the University of Tokyo.21 Electron density maps were drawn for the relevant molecular orbitals. The density of the nth line from the outside (d,) is k X 2" X a ~ - where ~ , k = 2.5 for C 0 2 , k = 0.3 for HCHO, and k = 0.5 for other molecules. The repulsive molecular surfaces were estimated from the van der Waals radii of atoms (rc = 1.70 A, ro = 1.40 A, rH = 1.20 A). Simple drawings of molecular orbitals were obtained from MO coefficients with circles or couples of ellipses whose areas are proportional to the square of the coefficients. In-plane p-type orbitals were shown by couples of ellipses. The two components of in-plane p-type orbitals were combined into a p orbital with a direction determined from the two respective coefficients. The out-of-plane component of p orbitals was shown by a broken circle. Valence s orbitals were shown by a solid circle. Signs of orbital coefficients were indicated by the thickness of the curves; thick curves are for a positive sign, and thin curves for a negative sign. For quantitative analyses of PIES band intensities, exterior electron densities (EED) were calculated; EED is defined for each M O as electron densities integrated outside the repulsive molecular surface (RMS).4 The R M S was approximated with spheres of van der Waals radii of constituent atoms, and it was further refined by covering valleys between adjacent spheres with common tangent planes. This modification was found to be necessary to avoid unfavorable valleys on the van der Waals surface, since the real repulsive molecular surface must be more smooth and cannot have such valleys. Numerical integration was carried out for the region between the RMS and an outer surface with a separation of 2.0 A from the RMS. Contributions from the outer region farther than 2.0 A from the R M S are negligible, as shown for primitive functions elsewhere.22 Results and Discussion Transmission-corrected He*(2%) PIES and He I UPS are shown in Figures 1-7. Bands are labeled by numbers from the left as well as respective molecular orbitals from which an electron is extracted upon ionization. Contour lines of electron densities for some molecular orbitals are shown together with a thick solid curve indicating repulsive molecular surface through which metastable atoms cannot penetrate into the inside. Thick dashed curves in Figures 1 and 2 indicate the repulsive molecular surface refined with common tangent planes. Under the experimental conditions, contributions due to acid dimers are negligible. The assignments shown in Figures 1-7 are in agreement with those given for UPS in recent ~ o r k . ~The ~ only - ~ ~exceptional (21) Kosugi, N. Program GSCF2, Program Library, The Computer Center, The University of Tokyo, Tokyo, Japan, 1981. (22) Ohno, K.; Ishida, T. Inr. J . Quantum Chem., to be published. (23) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies: Tokyo, 1981. (24) Carnovale, F.; Livett, M. K.; Peel,J. B. J . Chem. Phys. 1979, 71, 255.
' 13
\
c02
1
I
3%
\
i
I
I
\
1 i
/
i i
I
/
I
\
i
6
0
2
T
a
L 2 0 ELECTRON ENERGY / eV Figure 1. Transmission-corrected He*(2%) Penning ionization electron spectrum (PIES) and H e I ultraviolet photoelectron spectrum (UPS)for COz. Electron density maps are drawn for the relevant molecular orbitals (upper). Simple MO drawings are also shown (lower).
6
9
7
5
3
1
HCHO
% 11
9 7 ' 5 ' EL EC T R ON ENERGY / e V
Ib,
3
Figure 2. Transmission-corrected He*(23S) PIES and H e I UPS for HCHO.
case is the assignments for bands 4 and 5 of CH,COOH. The 12a' orbital in our calculation involves a character of the sp hybridization on the hydroxyl oxygen atom. In an earlier (25) Carnovale, F.; Gan, T. H.; Peel, J. B. J . Electron Spectrosc. Relat. Phenom. 1980, 20, 53. ( 2 6 ) Von Niessen, W.; Bieri, G.; Asbrink, L. J . Electron Spectrosc. Relat. Phenom. 1980, 2 1 , 175.
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 2017
PIES of Aldehydes and Carboxylic Acids
PIES 9
5
7
3
CH3CH0
He I UPS I
I
I
I
I
,
I
9 7 5 ELECTRON ENERGY / eV
11
Figure 3. Transmission-correctedHe*(23S) PIES and He I UPS for
CH3CH0.
9
,
I
9
I
I
7
I
I
5
I
3
ELECTRON ENERGY / eV Figure 5. Transmission-correctedHe*(23S) PIES and He I UPS for
HCOOH.
5" 3
7
9
5
7
3
13a'
4 ,
1
11
9
7
5
ELECTRON ENERGY
11
3
/
I
,
9
I
7
,
,
,
,
3
5
ELECTRON ENERGY
eV
Figure 4. Transmission-corrected He*(2%) PIES and He I UPS for
,
/
eV
Figure 6. Transmission-correctedHe*(23S) PIES and He I UPS for
CHSCH2CHO.
CH3COOH.
the corresponding orbital with the same character is the l l a ' orbital. Since our calculation gives a slightly better S C F energy of -227.4680 hartree in comparison with the earlier value of -227.4667 hartree, the following discussion is made on the basis of the present calculation. Carbon dioxide is a simple molecule containing the CO double bond. PIES of this molecule were measured by various group^.^'-^^
UPS of C 0 2 were also studied by many groups, and the band assignments were ~ e l l - e s t a b l i s h e d . ~In~ earlier PIES studies, vibrational structures for the second band (la,) were suggested to be different from those of UPS.27i28A later study by Hotop et al.29revealed that the Franck-Condon progressions were very similar between the two types of ionization and that the earlier results leading to the erroneous conclusions were due to the low resolution of the spectra contaminated with components due to
(27) c e r m l k , V. J . Electron Spectrosc. Relat. Phenom. 1976, 9, 419. (28) Brion, C. E.; Yee, D. S. C. J . Electron Spectrosc. Relat. Phenom. 1977, 12, 71.
(29) Hotop, H.;Kolb, E.; Lorenzen, J. J . Electron Spectrosc. Relat. Phenom. 1979, 16, 213.
2018 The Journal of Physical Chemistry, Vol. 90, No. 10. 1986
Ohno et al.
-SCH2COOH 7
He1 UPS
J V 11
Figure 8. Correlation diagram of orbitals for aldehydes.
9 7 5 3 ELECTRON ENERGY / eV
Figure 7. Transmission-correctedHe*(2%) PIES and He I UPS for CH,CH,COOH.
He*(2IS). The resolution of the present PIES was quite satisfactory. The result in Figure I gives a confirmation of the later conclusion by Hotop et al. Highly resolved features of the present PIES are summarized as follows. (i) In Figure 2, the maximum of band 1 of H C H O is very sharp and assigned to the vibrational component of u’ = 0. This indicates that the rather diffuse band envelope in PIES is not necessarily due to the deformation of the Franck-Condon progressions as suggested by Ye and Brion.28 Since the electronic transition in the Penning process can occur at various interparticle distances, the bandshapes also depend on the interparticle pot e n t i a l ~ . ~(ii) ~ Band 4 of H C H O and band 4 of HCOOH are clearly resolved in our PIES. (iii) Bands 3-8 of CH3COOH are well-resolved. One of the most important features of Penning ionization electron spectroscopy can be seen when the PIES intensities are compared with the spatial distributions of the corresponding molecular orbitals. In Figure 1, bands for 3u, and 4ugorbitals are apparently very much more enhanced in PIES than the bands ” This tendency is connected with electron for IT, and 1 ~orbitals. distributions of those u orbitals which are extending outside of the CO axis and easily attacked by metastable helium atom. This nature of the 3u, and 4u, orbitals is ascribed to a lone-pair character of these orbitals in the oxygen atoms; although these orbitals have some bonding character between the carbon atom and the oxygen atoms, atomic orbitals on the oxygen atoms as a whole behave as an sp hybrid orbital extending outside of the C O axis. Such an effect of orbitals with a lone-pair character or an sp hybridization character on PIES intensities has also been found for molecules containing the C N group for which a C N bonding u orbital gives a very much enhanced band in PIES of every nitrile compound (e.g., 7a, orbital of CH3CN).16 The present results thus indicate that such a feature of PIES-active orbitals may be transferred throughout similar compounds with the same functional group. ~~
(30) Niehaus, A. Ber. Bunsenges. Phys. Chem. 1973, 77, 632.
In the spectra of aldehydes (Figures 2-4), bands at about 4 eV in electron energy are enhanced in PIES. In the case of molecules containing the COOH group (Figures 5-7), bands at about 2.5 eV are very much enhanced in PIES. The special activity of the corresponding orbitals can be explained from their exterior electron distributions. The exterior electron distribution of the 5a, orbital for formaldehyde (Figure 2) is similar to the 3~7,and 4ugorbitals of COz and extending outside the C O axis. For other aldehydes, a correlation diagram in Figure 8 is helpful to make further analyses. The strongly enhanced band 6 of C H 3 C H 2 C H 0 (Figure 4) can be ascribed to the loa’ orbital having the sp hybridization character on the oxygen atom. In the case of acetaldehyde (Figure 3), bands 4 and 6 are enhanced in PIES, and both of the corresponding orbitals, 8a’ and 7a’, show the characteristic electron distribution on the oxygen atom. As can be seen from Figure 8, the 8a’ orbital of C H 3 C H 0 has a similar character as the 5al orbital of HCHO. However, the direction of the 8a’ orbital is obviously tilted from the C O bond axis so that the exterior electron distribution is rather suppressed. The ?a’ orbital of CH3CHO has a p character extending outside the CO axis as well as a pseudo-r-type bonding character similar to the lbz orbital of HCHO. It is noted that the former contribution is larger in the 7a‘ orbital of C H 3 C H 0 than the 9a’ orbital of CH3CH2CH0. The PIES of HCOOH is almost simply related to those for H C H O and H 2 0 (Figure 9). The correlation is very clear not only from the band positions but also from the relative intensities and bandshapes. It must be noted that the 8a’ orbital of HCOOH corresponding to the most enhanced band 5 (Figure 5) has a large electron density on the carbonyl oxygen atom and shows a good isolobal analogy with the 5al orbital of H C H O (Figure 2). For CH3COOH and CH3CH2COOH(Figures 6 and 7), very much enhanced bands in PIES are uniquely assigned to 9a’ and 1la’ orbitals, respectively. These orbitals have large exterior electron distributions on the C=O group like the 8a’ orbital of HCOOH. Although the 12a’ orbital of CH3CH2COOHhas also an sp hybridization character at the oxygen atom on the C=O bond, such a character is much more concentrated on the 1 la’ orbital. Among other bands for these molecules, band 4 of CH3COOH and band 6 of CH3CH2COOHare also clearly assigned to 12a’ and 13a’ orbitals, respectively, since the sp hybridization character on the oxygen atom of the O H group explains
PIES of Aldehydes and Carboxylic Acids
p--q + ’bl
H\ 0
30,
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 2019 TABLE I: EED for CO,
.d’
H’
I
2h
MO” In, In,
basis set 4-31G 4-31’Gb 0.0284 0.0283 0.0238
0.0237
MO” 3a, 4a,
basis set 4-31G 4-31’Gb 0.0184 0.0327 0.0171
0.0324
“As for degenerate orbitals (lng, ln,), the EED value for a single moiety is listed. bTheconstraint imposed on the exponents (s = p ) was removed, and modified exponents were used for the outer 2s primitive functions (see text).
o=c
i’
TABLE II: EED for HCHO and HCOOH
HCHO 2b2 (0.0355),lb, (0.0324), 5al (0.0472), 1b2 (0.0232) HCOOH loa’ (0.0260), 2a” (0.0271), 9a’ (0.0338), la” (0.0259), 8a’ (0.0359), 7a’ (0.021 1)
‘0 H’
b &/
I
/50,,;
I
10
8
6 1 2 0 ELECTRON ENERGY / e V
Figure 9. Correlation of PIES for H20, HCHO, and HCOOH.
the slight enhancement of the corresponding bands in PIES. In the case of CH,COOH, a slight enhancement of band 7 (loa’ orbital) is also ascribed to the sp hybridization character on the O H group. Although the above conclusions were derived from experimental characteristics of PIES intensities and isolobal characters of molecular orbitals, a simple theoretical model may be used for the more quantitative analyses; in the exterior electron model,4 relative PIES intensities are proportional to the exterior electron densities (EED) of the relevant molecular orbitals. There are, however, some problems to make such quantitative analyses extensively. (i) Exterior electron distributions are sensitive to the choice of basis functions used in a b initio S C F MO calculation^.^^ Particularly, EED values for nonbonding orbitals are considerably underestimated with small size basis sets. This tendency must be especially crucial for such systems containing the C 4 group. (ii) Repulsive molecular surfaces representing correct interparticle potentials are very difficult to be determined. Although the usual van der Waals radii were successfully used for hydrocarbon molecules in the estimation of the repulsive molecular surface^,^ such an approximation may hold some errors when the interparticle potentials are highly attractive. This is likely to be the case for the entrance channel of the potential curves, especially around oxygen atoms of the target molecules.29 Very recently, it was shown that one of the most economical ways to improve a conventional basis set is to modify the orbital exponents for the outermost s-type primitive functions by replacing them with those of smaller values; in the case of NH3, the EED for the nonbonding orbital showed a maximum value when the exponent of the outer 2s primitive function for a N atom in the 4-31G set was decreased to 0.50 times the standard value, and the EED values for the modified 4-3 1 G set (4-3 1’G) were found (31) Ohno, K.; Matsumoto, S.; Harada, Y . J . Chem. Phys. 1984,81, 2183.
to be a good approximation of those obtained for a near-Hartree-Fock M0.2Z The successful results of the 4-31’G set can be ascribed to the removal of the constraint imposed on the exponents (s = p ) in the original 4-31G set. In the present study, EED values were calculated with the 4-31’G set for COz, HCHO, and HCOOH. In Table I, EED values for orbitals of C02 are shown for the original 4-31G set and the modified set (4-31’G). The optimum exponents of the outer 2s functions for C and 0 atoms were found to be 0.64 times the standard values. In the 4-31G set, EED values for u orbitals are smaller than 7r orbitals, contrary to the experiment that the u orbitals are more active in PIES than the 7r orbitals. This disagreement is obviously due to the drawback of the original 4-31G set in describing nonbonding orbitals.31 The 4-31’G set, on the other hand, yielded satisfactory results: EED(a) > EED(7r). Table I1 lists EED values obtained with the 4-31’G set for HCHO and HCOOH. The modified exponents of the outer 2s functions were 0.336 times the standard values. The 5a, orbital, which is most active in PIES for HCHO, gives EED = 0.0472 which is much larger than those for other orbitals (0.023-0.036). The most active orbital in PIES for HCOOH, the 8a’ orbital, yields the largest EED among the occupied molecular orbitals. The calculation also indicates that the 9a’ orbital is the second most active orbital in PIES, in good agreement with experiment. For the more quantitative analyses, one may again point out that the following two factors are to be considered most important: (i) basis-sensitive characteristics of ab initio MO functions must be studied in more detail, and (ii) repulsive molecular surfaces representing correct interparticle potentials must also be studied. Finally, the following concluding remarks can be made on the basis of the present experiments. (1) In all molecules containing the C=O group, orbitals having the sp hybridization character on the oxygen atoms showed remarkable activities on the electrophilic attacks by metastable helium atoms. This indicates that exterior electron distributions outside the repulsive surface of the molecule are large enough for the relevant orbitals with the sp hybridization character to yield Penning electrons at high probabilities. (2) The isolobal analogy of the target orbitals in the reactivity is highly transferrable between the molecules containing the C=O group. Such an isolobal analogy holds not only for the carbonyl group but also for the hydroxyl group for which some orbitals having the sp hybridization character on the oxygen atom give large activities in Penning ionization.
Acknowledgment. We thank Professor Y. Harada for helpful comments, K. Imai for his help in the experiments, Y. Itoh for his assistance in the preparation of the simple MO drawings, and T. Ishida for his help in EED calculations. Registry No. C02, 124-38-9; HCHO, 50-00-0; CH,CHO, 75-07-0; CHpCH2CH0, 123-38-6; HCOOH, 64-18-6; CHjCOOH, 64-19-7; CH,CH,COOH, 79-09-4.
