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Conformational analysis of propionaldehyde (propanal) by two-photon spectroscopy of the 3s ← n Rydberg transition
8 August 1997
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 274 (1997) 354-360
Conformational analysis of propionaldehyde (propanal) by two-photon spectroscopy of the 3s n Rydberg transition Neil C. Shand, Chang-L. Ning, Josef Pfab Department of Chemist~', Heriot-Watt Universi~'. Riccarton, Edinburgh EH14 4AS, UK
Received 11 April 1997
Abstract The two-photon 3s ~ n Rydberg spectra of propionaldehyde have been recorded under cell and free-jet expansion conditions. The occurrence of two separate electronic systems for the cis and disfavoured gauche conformers has been confirmed. In-phase electronic transitions are found in both formyl and methyl torsional coordinates, and the analysis of torsional sequences has provided new information on conformational potential functions. The cis (n0 3s) conformer is stabilised, the energy gap between cis and gauche conformers increases to 1260 cm-t and the methyl internal rotation barriers increase for both conformers on electronic excitation. © 1997 Elsevier Science B.V.
1. Introduction The aliphatic aldehydes are known air pollutants whose one- and two-photon electronic spectra in the ultraviolet are relevant in conjunction with the remote sensing and the trace analysis of air pollutants with pulsed lasers [1-3]. The lowest Rydberg states of aliphatic aldehydes are of (n, 3s) character and give rise to two-photon spectra in the 340 to 390 nm [4] range. Resonance-enhanced multiphoton ionization (REMPI) spectra via two-photon resonant (n, 3s) Rydberg levels of organic carbonyl compounds have become interesting for the conformational analysis of aldehydes and ketones in conjunction with the technique of supersonic jet-cooling [5-8]. The conformational behaviour of acetaldehyde in both the ground and (n, 3s) Rydberg states is well established [8]. Both states prefer eclipsed dihedral conformations, and the electronic transition is accompanied by a significant increase in the barrier to
internal rotation. The situation is much less clear for propanal where mass-selectively detected REMPI spectra of the jet-cooled molecule have indicated that there are two electronic origins in the 3s ~ n region reflecting a cis and a skew excited state conformer [7]. The conformations of propanal in its ground electronic state have been studied extensively. Fig. 1 shows the Newman projections of the two stable and the intermediate unstable rotamers along the central C - C bonds. Potential functions for internal rotation of the aldehyde group have been deduced from microwave, IR and electron diffraction measurements as well as quantum chemical calculations. Energy differences A U' between the preferred cisand the energetically disfavoured gauche-rotamers ranging from 276 to 420 cm-1 and different potential functions V(q~) hindering internal rotation of the aldehyde group have been deduced from microwave and far-infrared measurements [9].
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-2614(97)00685-4
N. C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360 o
o H
H
~H
o
o
CH 3
H
H
H
355
and boxcar averager which were triggered by a photodiode and light pulses derived from the dye laser.
CH 3
~=0°
0=60 ~
O= 120"
qb= 180"
"s-cis"
"gauche"
"gauche"
"a-trans"
Fig. 1. Newman projections of propanal along the central C - C bond as a function of the dihedral angle qL The s-cis and q~ = 120° gauche conformers form the stable minima whereas the s-trans and @ = 60 ° gauche rotamers correspond to the maxima in the formyl torsional potential curve shown in Fig. 5.
We report here a re-examination of the (2 + 1) REMPI spectrum of jet-cooled propanal and its unusual temperature dependence. The vibrational analysis of the spectra has confirmed the existence of two distinct conformer-specific vibronic band systems and has provided new information on the large amplitude motions and the associated potential functions of the (n, 3s) state.
3. Results and analysis The resonances observed in the MPI spectra of propanal in the 340 to 390 nm range of the near-UV arise from the two-photon excitation of the 3s ~ n Rydberg transition. A third photon provides sufficient energy for ionization of the parent with a lowest I.E. of 9.97 eV [12]. The MPI fragmentation of propanal with nanosecond UV laser pulses can produce ion fragments ranging from C - to the molecular ion via competing channels. The branching into different channels depends on the laser wavelength and intensity [6,13]. Ambient temperature (n, 3s) REMPI spectra closely resemble VUV one-photon absorption spec-
2. Experimental The experimental set-up has been described before [8,10]. It consists of an in-house constructed vacuum chamber and appropriate flanges machined from aluminium, a XeC1 excimer pumped dye laser system and a set-up permitting the optogalvanic detection of Ar resonance lines for wavelength calibration. Pulsed jet-cooling of propanal vapour was achieved with a solenoid valve (Iota I, General Valve) with 0.5 mm nozzle diameter using backing pressures up to 800 Torr with He and Ar as cartier gases. The beam of the dye laser was focused with a planoconvex spherical silica lens of 30 cm focal length ca. 8 mm downstream from the orifice. The photoion current was collected by an arrangement of parallel plates held at a distance of 2.8 cm and charged to + 90 V and - 9 0 V, respectively from a series of battery packs. The arrangement of the pulsed supersonic jet valve and collection plates used is shown in detail in Fig. 2. The photoion current was detected with a differential amplifier of the type described by Adams et al. [11] and a gated integrator
rI
'
'h
Fig. 2. Pulsed solenoid valve and parallel plate assembly fixed to PVC block i for detection of the photoion current, a: sample inlet; b: solenoid, c: slug; d: teflon poppet and spring; e: Pt plates; f: Ni plates; g: focal area of laser beam (enlarged); h: apertures or baffles in PVC block preventing scattered light from striking the metal electrodes; i: PVC block.
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
356
Table 1 Band positions and assignments of peaks observed in the ( 2 + 1 ) R E M P I spectrum of jet-cooled propanal ~/cm-I
Ag/cm-J
Assignment
Relative intensity
Comment
53448 53695 53943.9 53969.0 53978.2 53995.4 54023.8 54146.0 54172.1 54181.4 54197.8 54225.6 54349.6 54375.5 54384.8 54399.2 54428.5 54554.8 54758.8 54785.3 54807.3 54853.0 54877.0 55024.2 55056.6 55085.2 55142.6 55213.6 55282.0 55307.2
-496 - 249 0 25.1 34.3 51.5 79.9 202.1 228.2 237.5 253.9 281.7 405.7 431.6 440.9 455.3 484.6 610.9 814.9 841.4 863.4 909. l 933.1 1080.3 1112.7 1141.3 1198.7 1269.7 1338.1 1363.3
1520 150 00° cis 241 231 2422 2322 151 151 241 151 23[ 15~ 2422 151 2322 152 1502 241 1502 231 15o2 2422 15o2 2322 153 154 00° gauche C H 3 C H O 0o° 81 7oI / 1 1 5 101 9~ 6~
s vw vw vw vw s vw vw vw vw s vw vw vw vw m vw w vw vw vw vw vw vw w w w w
hot band hot band
a Progression in mode 15:55341.7, All features are doublets. b Progression in mode 15:55415.6, c Progression in mode 15:55484.9, d Progression in mode 15 : 55509.3,
hot hot hot hot
band band band band
hot hot hot hot
band band band band
hot hot hot hot
band band band band
hot band hot band
impurity
doublet, ~ b c a
55545.1 and 55750.8 cm - I . 55618.8 and 55821.6 cm -~ . 55677.8 and 55886.7 cm - j . 55713.9 and 55917.1 c m - ~.
tra [14]. Fig. 3 shows an example of 295 K REMPI vapour spectra revealing a hot band progression extending to low energy from the origin with intervals of 260 cm - l . This can be assigned to the CCO bending mode of the cis conformer in good agreement with infrared bands of the vapour observed at 255 cm - I or 271 cm -1 [15,16]. Table 1 summarises the two-photon wavenumbers and assignments of the essential features in the lowenergy portion of the (2 + 1) REMPI spectrum of propionaldehyde. Frequencies and some of the as-
Table 2 Comparison and description of F r a n c k - C o n d o n active ground and excited state vibrations a Sym.
Normal mode
v'/cm- 1
v " / c m - J [15]
v6 v7 v8 119 vl0 7.~11 v15 /"23
a' at a' ap a' a' a' a"
v24
a"
asym. CH 3 def. CH~ scissor formyl CH def. sym. CH 3 def. CH 2 wag in-phase CCC str. CCCO def. cis CH 3 torsion gauche CH 3 torsion cis CHO torsion
1363 1199 b 1141 1338 1270 1199 b 202 218 c 226 160 e
1468 1423 1398 1381 1339 1093 271 184 d 204 1359
a The vibrations listed are those of the cis conformer unless stated otherwise. b Average of doublet. c Derived from cis 231 transition. d Deduced from F" = 5.650 c m - I and V:~'= 768 c m - i, Cox et al.[ 18 - 20]. e Derived from gauche 231 transition.
signments are in agreement with the previous work [6], but relative intensities of spectra differ significantly. Total ion current spectra reported here have been recorded under conditions where the ionisation step is saturated and REMPI signals increase with the square of the laser power and are normalised accordingly. The analysis summarised in Table 1 is consistent with results obtained for deuterated species [6]. It confirms the band near 53945 cm-1 as the electronic origin of the 3s ~ n Rydberg system of the cis conformer and the assignment of the prominent 203 cm -1 progression to v'15, an a' CCCO bending mode of the cis rotamer. The vibrations most active in the REMPI spectra are summarised in Table 2 together with their normal mode descriptions. Features previously ascribed to v 7, a CH 2 scissor vibration, appear as doublets which we attribute to a Fermi resonance between 1.,7 and v~.
Table 3 Comparison of methyl internal rotation parameters for cis and gauche propanal in the ground and (n, 3s) Rydberg state
cis gauche
F,,/cm -l
V~'/cm - i
Vi/cm -I
V~/cm -I
5.650 6.014
768 886
1292 1070
- 85 -
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
_ o04
1,o,,0,,oll
53500
54000
54500
,.
55000
Wavenumber
357
A
55500
56000
56500
/ c r n "1
Fig. 3. Ambient temperature (295 K) REMPI spectrum of propanal vapour with assignments of the cis 00° origin and progressions in //15, the a' CCCO skeletal bending mode of the cis rotamer. The most prominent band at 54785 c m - i is due to the gauche 00° origin and unresolved torsional sequence bands.
Their assignment to v 7 alone is neither consistent with their change to single features nor with their small shifts on deuteration. Other new assignments are a hot band progression in v15 and sequence bands in v23 and /224 appearing
as weak satellite features displaced to higher energy from the u~5 band origins associated with the cis conformer. The sequence bands provide important, new information on the torsional intervals and internal rotation barriers of the excited state. The ground
160
140
120
100 --~
80
a:
60
10% in He, 100 Ton"
40 ~
.
~
- L.
x
,_JL .
1
~.
2% in He, 300 Ton"
. . . .
20
54000
54500
.
~
55000
=
2% in He, 700 Ton.
55500
56000
Wavenumber / crn 1
Fig. 4. Comparison of REMPI spectra of 295 K propanal vapour and of samples jet-cooled under different expansion conditions showing the effects of temperature on origins and sequence band structure.
358
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
state torsional fundamentals are known or can be deduced from the known structure and torsional potentials. The methods used to calculate the torsional energy levels, internal rotation constants and potential barriers of propanal including the parameters for three-fold hindered internal rotation listed in Table 3 are described elsewhere [8,14,17]. Note the pronounced effect of temperature on a group of apparently isolated bands of which the one at 54785 cm-1 dominates the entire 295 K (n, 3s) REMPI spectrum shown in Fig. 3. The same band also dominates the one-photon VUV absorption spectra at ambient temperature [14]. Its assignment is of crucial importance for the conformational analysis of propanal. Metha et al. assigned this band to a skew - 00° origin, that is a transition originating from the less populated conformer with a skew dihedral arrangement [6]. Since this assignment implies a rather large energy difference of ca. 1260 c m between the cis and gauche conformers of the (n, 3s) Rydberg state, we have searched for alternatives that reduce A E' by assigning it to cis ~ gauche inter-well transitions in combination with an excited state vibration that makes it Franck-Condon allowed. An assignment of this type yielding an acceptable ground state energy difference A U ' = 357 cm-1 is the "hot combination band" denoted by 00~ 11~ where c and g denote the cis and gauche wells. We reject this and similar options, however, since torsions are only weakly active as sequence bands and entirely absent in cold spectra. For the same reason we exclude a transition from a gauche minimum to a barrier maximum. The observed (n, 3s) spectra must thus reflect vertical transitions from the cis and gauche minima to torsional potential curves of the (n, 3s) state that are in phase with that of the ground state. The second, " h o t " origin then mirrors a vertical Franck-Condon transition between gauche minima in both ground and excited states. Changes of the REMPI spectra with jet expansion conditions are drastic as shown in Fig. 4. Lowering the He stagnation pressure reduces cooling and leads to a marked increase of the second origin band and the emergence of blue shifted sequence transitions as weak satellite features accompanying all major vibronic bands. The band coinciding with the second origin increases rapidly and acquires a blue shifted sequence in the methyl torsion resembling similar
satellite features of the cis origin and u'15 bands. The second origin is reduced but survives as a weak feature on jet-cooling when Trot < 10 K. Residual population therefore remains trapped in the higher well of the gauche conformer. The marked intensity of this composite band in the 300 K vapour spectra suggests that the gauche/cis conformer ratio is larger than 0.26, that is two times the Boltzmann ratio at 298 K for A E " = 4 2 0 cm -1 Calculations of the thermal occupancies of the aldehyde torsional energy levels calculated for the ground state potential function of Fig. 5 show, however, that the population of the gauche wells is almost twice as high favouring
2000, 1800l
1600I
~
1400":] 1200 lOOO8oo~
~ . ,
1
I
l~
o ........................................................................................................... :
Q- 1200
1000 800 600 400 200 0- 180........ -90 , ....... 0
Torsionalangle ~/ o
90
180
Fig. 5. Schematic ground and excited state potentials along the formyl torsional coordinate @ showing vertical in-phase 3s ~ - n electronic transitions between the cis and gauche conformers.
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
the cis conformation only by a ratio of 6 to 4 [14]. The unusual temperature dependence of the second origin arises therefore from sequence transitions in the formyl torsion reflecting the population of gauche levels up to and above the top of the gauche-gauche barrier as shown in Fig. 5. The sum of these unresolved torsional hot bands outweigh the gauche 00° origin in intensity and make up its broad contour and pedestal at 295 K. Transitions between free rotor states encompassing both degenerate gauche conformers may cause the asymmetric red shift of its contour at high temperature. From the different displacements of the methyl torsional sequence bands to the blue of the cis and gauche origins we have deduced values of 218 c m - 1 and 226 cm-~ for the methyl torsional fundamentals in the (n, 3s) Rydberg state and the V3 barriers given in Table 3. The negative Vr' term necessary to reproduce the torsional levels indicates some coupling of aldehyde and methyl torsions in the (n, 3s) cis conformer.
4. Discussion Propanal constitutes a suitable model for studying the influence of a symmetric (CH 3) and of an asymmetric (CHO) internal rotor on the large amplitude vibrational structure of a Rydberg transition. Fig. 5 summarises the essential conclusions from this work about the conformational changes that accompany this 3s ~ n Rydberg transition. Vertical transitions from the cis and gauche wells take the molecule to an (n, 3s) torsional potential that is in-phase with the ground state potential. The minima corresponding to the cis and gauche conformers in the Rydberg state are separated by an energy difference of A E ' = 1260 cm -~, a nearly three-fold increase from the ground state value of A E" = 420 c m - 1.9 Since the transition is vertical between in-phase potential minima and involves no significant changes in the torsional coordinates 4) and a, the torsional modes of the formyl and methyl groups are only weakly active as 231, 24~1 and higher sequence transitions. The asymmetric (formyl) torsional potential deduced for the (n, 3s) Rydberg state shown in Fig. 5 is tentative, but compatible with an excited state torsional fundamental of 218 cm -~ for the cis conformer and the
359
observed A E ' = 1261 cm - l gauche-cis energy difference. The shape of the ground and (n, 3s) gauche wells cannot differ greatly since warming of the sample piles up intensity close to the gauche origin. A general red shift of unresolved gauche-gauche hot band transitions indicates that the gauche-gauche barrier decreases in the excited state as indicated schematically in Fig. 5. The height of the cis-gauche barrier in the (n, 3s) state is not well defined, and more information is needed to describe the asymmetric torsional potential in detail. The substitution of an eclipsed H atom in acetaldehyde by a methyl group has profound effects on the formyl torsional potential function of the Rydberg state and stabilises the cis well with respect to the gauche well. This confirms the expectation that attractive forces due to the intramolecular dipole-induced dipole interactions increase on excitation of the (n, 3s) Rydberg state. Barriers listed in Table 3 for the internal rotation of the methyl groups in both conformers are deduced from the observed methyl sequence structure and known ground state data [18-20]. Both V 3 barriers increase on electronic excitation, but the increase is significantly larger for the cis than for the gauche conformer indicating that dipole-induced dipole attractions between the methyl rotors and the frame also favour the cis conformer. Microwave measurements have shown that jetcooling freezes out the higher conformers of propanal and that conformers are not generally relaxed by Ar if they are separated by barriers larger than about 350 cm -1 [21]. Propanal was classified as an intermediate, weakly relaxing case, and this is confirmed by the present REMPI study. It is mainly the disappearance of gauche-gauche sequence bands and not conformer relaxation that causes the collapse, on jet-cooling, of the strong feature coinciding with the gauche-gauche origin. A similar case of conformer relaxation has been studied in glyoxal by the laserinduced fluorescence (LIF) technique [22].
Acknowledgements We thank Dr. K.P. Lawley for his interest and for providing a computer program and Dr. A.P. Cox for helpful discussions. NCS thanks the EPSRC for an earmarked studentship. This work was supported by
360
N.C. Shand et al. / Chemical Physics Letters 2 74 (1997) 354-360
the Atmospheric Chemistry Initiatives of the UK Science and Engineering Research Council (SERC) and Natural Environment Research Council (NERC).
References [1] R.P. Wayne, Chemistry of Atmospheres, Clarendon Press, Oxford, 1991. [2] L. Thomas, Spectroscopy in Environmental Science, in: R.J.H. Clark, R.E. Hester (Eds.), Wiley, New York, 1995, p. 1. [3] J. Pfab, Spectroscopy in Environmental Science, in: R.J.H. Clark, R.E. Hester (Eds.), Wiley, New York, 1995, p. 149. [4] B.A. Heath, M.B. Robin, N.A. Kuebler, G.J. Fisanick, T.S. Eichelberger, J. Chem. Phys. 72 (1980) 5565. [5] T.J. Cornish, T. Baer, J. Phys. Chem. 94 (1990) 2852. [6] G.F. Metha, M.A. Buntine, D.C. McGilvery, R.J.S. Morrison, J. Mol. Spectry. 165 (1994) 32. [7] H. Gu, T. Kundu, L. Goodman, J. Phys. Chem. 97 (1993) 7194. [8] N.C. Shand, C.L. Ning, J. Pfab, Chem. Phys. Lett. 247 (1995) 32. [9] J. Randell, J.A. Hardy, A.P. Cox, J. Chem. Soc. Faraday Trans. 2 (84) (1988) 1199.
[10] M. Hippler, Ph.D. Thesis, Heriot-Watt University, Edinburgh (1993). [11] T.E. Adams, R.J.S. Morrison, E.R. Grant, Rev. Sci. Instrum. 51 (1980) 1414. [12] W. Tam, D. Lee, C.E. Brion, J. Electron Spectry. 4 (1974) 77. [13] H.X. Liu, Z.L. Li, S.T. Li, Y.L. Qian, Y. Wang, C.K. Wu, Chem. Phys. 129 (1989) 495. [14] N.C. Shand, C. Ning, I.C. Walker, M.R.F. Siggel, J. Pfab, J. Chem. Soc. Faraday Trans. 93 (1997), in press. [15] S.G. Frankiss, W. Kynaston, Spectrochim. Acta 28 a (1972) 2149. [16] P. van Nuffel, L. van den Enden, C. van Alsenoy, H. Geise, J. Mol. Struct. 116 (1984) 99. [17] K.P. Lawley, J. Mol. Spectry. 183 (1997) 25. [18] J. Randell, A.P. Cox, H. Dreizler, Z. Naturforsch. 42 a (1987) 957. [19] J. Randell, A.P. Cox, K.W. Hillig, M. Imachi, M.S. LaBarge, R.L. Kuczowski, Z. Naturforsch. 43 a (1988) 27l. [20] A.P. Cox, personal communication. [21] R.S. Ruoff, T.D. Klotz, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 93 (1990) 3142. [22] K.W. Butz, D.J. Krajnovich, C.S. Parmenter, J. Chem. Phys. 93 (1990) 1557.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 274 (1997) 354-360
Conformational analysis of propionaldehyde (propanal) by two-photon spectroscopy of the 3s n Rydberg transition Neil C. Shand, Chang-L. Ning, Josef Pfab Department of Chemist~', Heriot-Watt Universi~'. Riccarton, Edinburgh EH14 4AS, UK
Received 11 April 1997
Abstract The two-photon 3s ~ n Rydberg spectra of propionaldehyde have been recorded under cell and free-jet expansion conditions. The occurrence of two separate electronic systems for the cis and disfavoured gauche conformers has been confirmed. In-phase electronic transitions are found in both formyl and methyl torsional coordinates, and the analysis of torsional sequences has provided new information on conformational potential functions. The cis (n0 3s) conformer is stabilised, the energy gap between cis and gauche conformers increases to 1260 cm-t and the methyl internal rotation barriers increase for both conformers on electronic excitation. © 1997 Elsevier Science B.V.
1. Introduction The aliphatic aldehydes are known air pollutants whose one- and two-photon electronic spectra in the ultraviolet are relevant in conjunction with the remote sensing and the trace analysis of air pollutants with pulsed lasers [1-3]. The lowest Rydberg states of aliphatic aldehydes are of (n, 3s) character and give rise to two-photon spectra in the 340 to 390 nm [4] range. Resonance-enhanced multiphoton ionization (REMPI) spectra via two-photon resonant (n, 3s) Rydberg levels of organic carbonyl compounds have become interesting for the conformational analysis of aldehydes and ketones in conjunction with the technique of supersonic jet-cooling [5-8]. The conformational behaviour of acetaldehyde in both the ground and (n, 3s) Rydberg states is well established [8]. Both states prefer eclipsed dihedral conformations, and the electronic transition is accompanied by a significant increase in the barrier to
internal rotation. The situation is much less clear for propanal where mass-selectively detected REMPI spectra of the jet-cooled molecule have indicated that there are two electronic origins in the 3s ~ n region reflecting a cis and a skew excited state conformer [7]. The conformations of propanal in its ground electronic state have been studied extensively. Fig. 1 shows the Newman projections of the two stable and the intermediate unstable rotamers along the central C - C bonds. Potential functions for internal rotation of the aldehyde group have been deduced from microwave, IR and electron diffraction measurements as well as quantum chemical calculations. Energy differences A U' between the preferred cisand the energetically disfavoured gauche-rotamers ranging from 276 to 420 cm-1 and different potential functions V(q~) hindering internal rotation of the aldehyde group have been deduced from microwave and far-infrared measurements [9].
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-2614(97)00685-4
N. C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360 o
o H
H
~H
o
o
CH 3
H
H
H
355
and boxcar averager which were triggered by a photodiode and light pulses derived from the dye laser.
CH 3
~=0°
0=60 ~
O= 120"
qb= 180"
"s-cis"
"gauche"
"gauche"
"a-trans"
Fig. 1. Newman projections of propanal along the central C - C bond as a function of the dihedral angle qL The s-cis and q~ = 120° gauche conformers form the stable minima whereas the s-trans and @ = 60 ° gauche rotamers correspond to the maxima in the formyl torsional potential curve shown in Fig. 5.
We report here a re-examination of the (2 + 1) REMPI spectrum of jet-cooled propanal and its unusual temperature dependence. The vibrational analysis of the spectra has confirmed the existence of two distinct conformer-specific vibronic band systems and has provided new information on the large amplitude motions and the associated potential functions of the (n, 3s) state.
3. Results and analysis The resonances observed in the MPI spectra of propanal in the 340 to 390 nm range of the near-UV arise from the two-photon excitation of the 3s ~ n Rydberg transition. A third photon provides sufficient energy for ionization of the parent with a lowest I.E. of 9.97 eV [12]. The MPI fragmentation of propanal with nanosecond UV laser pulses can produce ion fragments ranging from C - to the molecular ion via competing channels. The branching into different channels depends on the laser wavelength and intensity [6,13]. Ambient temperature (n, 3s) REMPI spectra closely resemble VUV one-photon absorption spec-
2. Experimental The experimental set-up has been described before [8,10]. It consists of an in-house constructed vacuum chamber and appropriate flanges machined from aluminium, a XeC1 excimer pumped dye laser system and a set-up permitting the optogalvanic detection of Ar resonance lines for wavelength calibration. Pulsed jet-cooling of propanal vapour was achieved with a solenoid valve (Iota I, General Valve) with 0.5 mm nozzle diameter using backing pressures up to 800 Torr with He and Ar as cartier gases. The beam of the dye laser was focused with a planoconvex spherical silica lens of 30 cm focal length ca. 8 mm downstream from the orifice. The photoion current was collected by an arrangement of parallel plates held at a distance of 2.8 cm and charged to + 90 V and - 9 0 V, respectively from a series of battery packs. The arrangement of the pulsed supersonic jet valve and collection plates used is shown in detail in Fig. 2. The photoion current was detected with a differential amplifier of the type described by Adams et al. [11] and a gated integrator
rI
'
'h
Fig. 2. Pulsed solenoid valve and parallel plate assembly fixed to PVC block i for detection of the photoion current, a: sample inlet; b: solenoid, c: slug; d: teflon poppet and spring; e: Pt plates; f: Ni plates; g: focal area of laser beam (enlarged); h: apertures or baffles in PVC block preventing scattered light from striking the metal electrodes; i: PVC block.
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
356
Table 1 Band positions and assignments of peaks observed in the ( 2 + 1 ) R E M P I spectrum of jet-cooled propanal ~/cm-I
Ag/cm-J
Assignment
Relative intensity
Comment
53448 53695 53943.9 53969.0 53978.2 53995.4 54023.8 54146.0 54172.1 54181.4 54197.8 54225.6 54349.6 54375.5 54384.8 54399.2 54428.5 54554.8 54758.8 54785.3 54807.3 54853.0 54877.0 55024.2 55056.6 55085.2 55142.6 55213.6 55282.0 55307.2
-496 - 249 0 25.1 34.3 51.5 79.9 202.1 228.2 237.5 253.9 281.7 405.7 431.6 440.9 455.3 484.6 610.9 814.9 841.4 863.4 909. l 933.1 1080.3 1112.7 1141.3 1198.7 1269.7 1338.1 1363.3
1520 150 00° cis 241 231 2422 2322 151 151 241 151 23[ 15~ 2422 151 2322 152 1502 241 1502 231 15o2 2422 15o2 2322 153 154 00° gauche C H 3 C H O 0o° 81 7oI / 1 1 5 101 9~ 6~
s vw vw vw vw s vw vw vw vw s vw vw vw vw m vw w vw vw vw vw vw vw w w w w
hot band hot band
a Progression in mode 15:55341.7, All features are doublets. b Progression in mode 15:55415.6, c Progression in mode 15:55484.9, d Progression in mode 15 : 55509.3,
hot hot hot hot
band band band band
hot hot hot hot
band band band band
hot hot hot hot
band band band band
hot band hot band
impurity
doublet, ~ b c a
55545.1 and 55750.8 cm - I . 55618.8 and 55821.6 cm -~ . 55677.8 and 55886.7 cm - j . 55713.9 and 55917.1 c m - ~.
tra [14]. Fig. 3 shows an example of 295 K REMPI vapour spectra revealing a hot band progression extending to low energy from the origin with intervals of 260 cm - l . This can be assigned to the CCO bending mode of the cis conformer in good agreement with infrared bands of the vapour observed at 255 cm - I or 271 cm -1 [15,16]. Table 1 summarises the two-photon wavenumbers and assignments of the essential features in the lowenergy portion of the (2 + 1) REMPI spectrum of propionaldehyde. Frequencies and some of the as-
Table 2 Comparison and description of F r a n c k - C o n d o n active ground and excited state vibrations a Sym.
Normal mode
v'/cm- 1
v " / c m - J [15]
v6 v7 v8 119 vl0 7.~11 v15 /"23
a' at a' ap a' a' a' a"
v24
a"
asym. CH 3 def. CH~ scissor formyl CH def. sym. CH 3 def. CH 2 wag in-phase CCC str. CCCO def. cis CH 3 torsion gauche CH 3 torsion cis CHO torsion
1363 1199 b 1141 1338 1270 1199 b 202 218 c 226 160 e
1468 1423 1398 1381 1339 1093 271 184 d 204 1359
a The vibrations listed are those of the cis conformer unless stated otherwise. b Average of doublet. c Derived from cis 231 transition. d Deduced from F" = 5.650 c m - I and V:~'= 768 c m - i, Cox et al.[ 18 - 20]. e Derived from gauche 231 transition.
signments are in agreement with the previous work [6], but relative intensities of spectra differ significantly. Total ion current spectra reported here have been recorded under conditions where the ionisation step is saturated and REMPI signals increase with the square of the laser power and are normalised accordingly. The analysis summarised in Table 1 is consistent with results obtained for deuterated species [6]. It confirms the band near 53945 cm-1 as the electronic origin of the 3s ~ n Rydberg system of the cis conformer and the assignment of the prominent 203 cm -1 progression to v'15, an a' CCCO bending mode of the cis rotamer. The vibrations most active in the REMPI spectra are summarised in Table 2 together with their normal mode descriptions. Features previously ascribed to v 7, a CH 2 scissor vibration, appear as doublets which we attribute to a Fermi resonance between 1.,7 and v~.
Table 3 Comparison of methyl internal rotation parameters for cis and gauche propanal in the ground and (n, 3s) Rydberg state
cis gauche
F,,/cm -l
V~'/cm - i
Vi/cm -I
V~/cm -I
5.650 6.014
768 886
1292 1070
- 85 -
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
_ o04
1,o,,0,,oll
53500
54000
54500
,.
55000
Wavenumber
357
A
55500
56000
56500
/ c r n "1
Fig. 3. Ambient temperature (295 K) REMPI spectrum of propanal vapour with assignments of the cis 00° origin and progressions in //15, the a' CCCO skeletal bending mode of the cis rotamer. The most prominent band at 54785 c m - i is due to the gauche 00° origin and unresolved torsional sequence bands.
Their assignment to v 7 alone is neither consistent with their change to single features nor with their small shifts on deuteration. Other new assignments are a hot band progression in v15 and sequence bands in v23 and /224 appearing
as weak satellite features displaced to higher energy from the u~5 band origins associated with the cis conformer. The sequence bands provide important, new information on the torsional intervals and internal rotation barriers of the excited state. The ground
160
140
120
100 --~
80
a:
60
10% in He, 100 Ton"
40 ~
.
~
- L.
x
,_JL .
1
~.
2% in He, 300 Ton"
. . . .
20
54000
54500
.
~
55000
=
2% in He, 700 Ton.
55500
56000
Wavenumber / crn 1
Fig. 4. Comparison of REMPI spectra of 295 K propanal vapour and of samples jet-cooled under different expansion conditions showing the effects of temperature on origins and sequence band structure.
358
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
state torsional fundamentals are known or can be deduced from the known structure and torsional potentials. The methods used to calculate the torsional energy levels, internal rotation constants and potential barriers of propanal including the parameters for three-fold hindered internal rotation listed in Table 3 are described elsewhere [8,14,17]. Note the pronounced effect of temperature on a group of apparently isolated bands of which the one at 54785 cm-1 dominates the entire 295 K (n, 3s) REMPI spectrum shown in Fig. 3. The same band also dominates the one-photon VUV absorption spectra at ambient temperature [14]. Its assignment is of crucial importance for the conformational analysis of propanal. Metha et al. assigned this band to a skew - 00° origin, that is a transition originating from the less populated conformer with a skew dihedral arrangement [6]. Since this assignment implies a rather large energy difference of ca. 1260 c m between the cis and gauche conformers of the (n, 3s) Rydberg state, we have searched for alternatives that reduce A E' by assigning it to cis ~ gauche inter-well transitions in combination with an excited state vibration that makes it Franck-Condon allowed. An assignment of this type yielding an acceptable ground state energy difference A U ' = 357 cm-1 is the "hot combination band" denoted by 00~ 11~ where c and g denote the cis and gauche wells. We reject this and similar options, however, since torsions are only weakly active as sequence bands and entirely absent in cold spectra. For the same reason we exclude a transition from a gauche minimum to a barrier maximum. The observed (n, 3s) spectra must thus reflect vertical transitions from the cis and gauche minima to torsional potential curves of the (n, 3s) state that are in phase with that of the ground state. The second, " h o t " origin then mirrors a vertical Franck-Condon transition between gauche minima in both ground and excited states. Changes of the REMPI spectra with jet expansion conditions are drastic as shown in Fig. 4. Lowering the He stagnation pressure reduces cooling and leads to a marked increase of the second origin band and the emergence of blue shifted sequence transitions as weak satellite features accompanying all major vibronic bands. The band coinciding with the second origin increases rapidly and acquires a blue shifted sequence in the methyl torsion resembling similar
satellite features of the cis origin and u'15 bands. The second origin is reduced but survives as a weak feature on jet-cooling when Trot < 10 K. Residual population therefore remains trapped in the higher well of the gauche conformer. The marked intensity of this composite band in the 300 K vapour spectra suggests that the gauche/cis conformer ratio is larger than 0.26, that is two times the Boltzmann ratio at 298 K for A E " = 4 2 0 cm -1 Calculations of the thermal occupancies of the aldehyde torsional energy levels calculated for the ground state potential function of Fig. 5 show, however, that the population of the gauche wells is almost twice as high favouring
2000, 1800l
1600I
~
1400":] 1200 lOOO8oo~
~ . ,
1
I
l~
o ........................................................................................................... :
Q- 1200
1000 800 600 400 200 0- 180........ -90 , ....... 0
Torsionalangle ~/ o
90
180
Fig. 5. Schematic ground and excited state potentials along the formyl torsional coordinate @ showing vertical in-phase 3s ~ - n electronic transitions between the cis and gauche conformers.
N.C. Shand et al. / Chemical Physics Letters 274 (1997) 354-360
the cis conformation only by a ratio of 6 to 4 [14]. The unusual temperature dependence of the second origin arises therefore from sequence transitions in the formyl torsion reflecting the population of gauche levels up to and above the top of the gauche-gauche barrier as shown in Fig. 5. The sum of these unresolved torsional hot bands outweigh the gauche 00° origin in intensity and make up its broad contour and pedestal at 295 K. Transitions between free rotor states encompassing both degenerate gauche conformers may cause the asymmetric red shift of its contour at high temperature. From the different displacements of the methyl torsional sequence bands to the blue of the cis and gauche origins we have deduced values of 218 c m - 1 and 226 cm-~ for the methyl torsional fundamentals in the (n, 3s) Rydberg state and the V3 barriers given in Table 3. The negative Vr' term necessary to reproduce the torsional levels indicates some coupling of aldehyde and methyl torsions in the (n, 3s) cis conformer.
4. Discussion Propanal constitutes a suitable model for studying the influence of a symmetric (CH 3) and of an asymmetric (CHO) internal rotor on the large amplitude vibrational structure of a Rydberg transition. Fig. 5 summarises the essential conclusions from this work about the conformational changes that accompany this 3s ~ n Rydberg transition. Vertical transitions from the cis and gauche wells take the molecule to an (n, 3s) torsional potential that is in-phase with the ground state potential. The minima corresponding to the cis and gauche conformers in the Rydberg state are separated by an energy difference of A E ' = 1260 cm -~, a nearly three-fold increase from the ground state value of A E" = 420 c m - 1.9 Since the transition is vertical between in-phase potential minima and involves no significant changes in the torsional coordinates 4) and a, the torsional modes of the formyl and methyl groups are only weakly active as 231, 24~1 and higher sequence transitions. The asymmetric (formyl) torsional potential deduced for the (n, 3s) Rydberg state shown in Fig. 5 is tentative, but compatible with an excited state torsional fundamental of 218 cm -~ for the cis conformer and the
359
observed A E ' = 1261 cm - l gauche-cis energy difference. The shape of the ground and (n, 3s) gauche wells cannot differ greatly since warming of the sample piles up intensity close to the gauche origin. A general red shift of unresolved gauche-gauche hot band transitions indicates that the gauche-gauche barrier decreases in the excited state as indicated schematically in Fig. 5. The height of the cis-gauche barrier in the (n, 3s) state is not well defined, and more information is needed to describe the asymmetric torsional potential in detail. The substitution of an eclipsed H atom in acetaldehyde by a methyl group has profound effects on the formyl torsional potential function of the Rydberg state and stabilises the cis well with respect to the gauche well. This confirms the expectation that attractive forces due to the intramolecular dipole-induced dipole interactions increase on excitation of the (n, 3s) Rydberg state. Barriers listed in Table 3 for the internal rotation of the methyl groups in both conformers are deduced from the observed methyl sequence structure and known ground state data [18-20]. Both V 3 barriers increase on electronic excitation, but the increase is significantly larger for the cis than for the gauche conformer indicating that dipole-induced dipole attractions between the methyl rotors and the frame also favour the cis conformer. Microwave measurements have shown that jetcooling freezes out the higher conformers of propanal and that conformers are not generally relaxed by Ar if they are separated by barriers larger than about 350 cm -1 [21]. Propanal was classified as an intermediate, weakly relaxing case, and this is confirmed by the present REMPI study. It is mainly the disappearance of gauche-gauche sequence bands and not conformer relaxation that causes the collapse, on jet-cooling, of the strong feature coinciding with the gauche-gauche origin. A similar case of conformer relaxation has been studied in glyoxal by the laserinduced fluorescence (LIF) technique [22].
Acknowledgements We thank Dr. K.P. Lawley for his interest and for providing a computer program and Dr. A.P. Cox for helpful discussions. NCS thanks the EPSRC for an earmarked studentship. This work was supported by
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N.C. Shand et al. / Chemical Physics Letters 2 74 (1997) 354-360
the Atmospheric Chemistry Initiatives of the UK Science and Engineering Research Council (SERC) and Natural Environment Research Council (NERC).
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[10] M. Hippler, Ph.D. Thesis, Heriot-Watt University, Edinburgh (1993). [11] T.E. Adams, R.J.S. Morrison, E.R. Grant, Rev. Sci. Instrum. 51 (1980) 1414. [12] W. Tam, D. Lee, C.E. Brion, J. Electron Spectry. 4 (1974) 77. [13] H.X. Liu, Z.L. Li, S.T. Li, Y.L. Qian, Y. Wang, C.K. Wu, Chem. Phys. 129 (1989) 495. [14] N.C. Shand, C. Ning, I.C. Walker, M.R.F. Siggel, J. Pfab, J. Chem. Soc. Faraday Trans. 93 (1997), in press. [15] S.G. Frankiss, W. Kynaston, Spectrochim. Acta 28 a (1972) 2149. [16] P. van Nuffel, L. van den Enden, C. van Alsenoy, H. Geise, J. Mol. Struct. 116 (1984) 99. [17] K.P. Lawley, J. Mol. Spectry. 183 (1997) 25. [18] J. Randell, A.P. Cox, H. Dreizler, Z. Naturforsch. 42 a (1987) 957. [19] J. Randell, A.P. Cox, K.W. Hillig, M. Imachi, M.S. LaBarge, R.L. Kuczowski, Z. Naturforsch. 43 a (1988) 27l. [20] A.P. Cox, personal communication. [21] R.S. Ruoff, T.D. Klotz, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 93 (1990) 3142. [22] K.W. Butz, D.J. Krajnovich, C.S. Parmenter, J. Chem. Phys. 93 (1990) 1557.