Infrared matrix spectroscopy of methylvinylether: Study of conformation interconversion by thermal molecular beams and by infrared irradiation

Spcctrochimica Acta, Vol. 41A, No. 1/2, pp. 319-339, 1985

0584-8539/85 $3.00 + 0.00 © 1985 Pergamon Press Ltd.

Printed in Great Britain.

Infrared matrix spectroscopy of methylvinylether: study of conformation interconversion by thermal molecular beams and by infrared irradiation TIMOTHY BEECH, R. GUNDE, P. FELDER and HS. GUNTHARD* Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH Zentrum, 8092 Ziirich, Switzerland (Received 25 June 1984) Abstract--Infrared matrix studies of two conformers (s-cis and s-gauche) of methylvinylether (MVE) will be reported. Thermal molecular beam spectra with Knudsen cell temperatures up to 925 K yield nearly complete assignments of the fundamentals of both conformers and enthalpy of conversion, AH ~ 6.62(35) kJ/mol. Irradiation of MVE:M (M = Ar, N2) by 3300-2800cm -1 radiation (but not by radiation with < 1450cm-1) induces photoconversion of s-cis to the same less stable conformer as produced by thermal beam technique; the conversion is shown to be a one-photon process. Quantum yield of conversion is about three times lower for MVE:N2 than to MVE:Ar. Highly complex band profiles were found in both matrices. By line shape analysis the band profiles are found to be complex superpositions of near Lorentzian-bands. Intensity of the components are shown to vary either with Knudsen cell temperature or with duration of photoeonversion. Line shape analysis is found useful for site structure discrimination and identification of vibrational transitions of the two conformers. Some features of the spectra indicate the less stable conformer to possess gauche re-structure.

induced spontaneous conformer interconversions of matrix isolated molecules were reported earlier [4, 5]. Conformer interconversion by i.r. irradiation was first observed with H O N O by HALL and PIMENTEL [6] and studied recently more closely in this case by SHIRK and McDONALD [7]. Since the first discovery the phenomen has been observed at various occasions, e.g. with glycol [8] and with haloethanols [9, 10, 11]. Recently it has been shown that the interconversion in the ease of fluoroethanol requires photons with energy comparable to the v(OH) stretching mode quantum[12]. Similar experiments on CH2FCH2OH were reported by SHIRK and HOFFMANN[13]. Also for CH2CICH2OH the efficiency of i.r. radiation for conformer interconversion has been demonstrated by TACHEUCHI and TASUMI [14]. In recent experiments aiming at the study of the conformers of methylvinylether (MVE) by thermal molecular beam matrix spectroscopy, this molecule was observed to experience conformer interconversion by globar irradiation. The high thermal stability of this molecule allows TMB studies at temperature up to 1000 K and it shows a pronounced susceptibility to i.r. conversion. Investigations will be described aiming at determination of photochemical aspects like energy of radiation required for conversion, relevance of one- or two-photon processes, population of sites not accessible by TMB experiments, etc. Though the MVE:Ar system (MVE isolated in Ar matrices) does not show hole burning in bands accessible to CO2 laser emissions, it proved appropriate for gaining further information about finer details of molecule-matrix interactions. TMB experiments with Knudsen cell temperatures in the range 298-922 K and with the

M E M O I R S O F D R . H . W. T H O M P S O N

Shortly after New Year 1948 H. W. THOMPSON gave three lectures for the staff and graduate students of the Organic Chemistry Laboratory on chemical i.r. spectroscopy and its applications to organic chemistry structural problems. His lectures created great enthusiasm for the potential and use of this then brand new instrumental technique. Indeed within a few years many structural problems in various fields like steroids, mono-, sesquiand triterpenes were solved by means of i.r. spectra. The impact of Tommy's lectures can scarcely be overestimated. They prepared the grounds for many of the later developments and the willingness of even the more conservative organic chemists of the OC Laboratory to use the rapidly growing family of spectroscopic techniques. 1. I N T R O D U C T I O N

Within recent years it has been shown that conformations of polyatomic molecules with K ~< 15 atoms may be studied rather systematically by combining thermal molecular beam techniques with i.r. matrix spectroscopy [1, 2]. Very recently i.r. matrix spectra generated by thermal molecular beams have been found to reveal unexpectedly complex band contours dependent on the beam source temperature, but not necessarily related to the existence of conformers [3]; the complexity of the matrix spectrum may equally well be related to the structure of the defects describing the state of a molecule in the rare gas matrix. Thermally *Author to whom correspondence should be addressed. 319

320

TIMOTHYBEECHet al.

spectrometer radiation restricted by interference filters to the interval 140&700 cm- ’ yield fairly well behaved Van? Hoff plots for bands in this region. Restriction of irradiation to the i.r. region < 1400 cm-’ does not induce conformer interconversion to a measurable extent, whereas subsequent exposure to broad band globar irradiation leads to fast interconversion. Radiation restricted to the 3300-2700 cm-’ region will be shown to produce interconversion to the unstable conformer and to lead to a photostationary state. From the dependence of the interconversion rate on irradiation fluence the process is concluded to be a one-photon process. Radiation induced interconversion rates for CHJOCH:CH2:Ar and CH,OCH:CH,:N, are found to be widely different. TMB experiments and photochemical interconversion generate distinctly differeni band profiles. In order to obtain improved data on these, many of the profiles were investigated by line shape analysis and a procedure for deconvolution based on line shape analysis reported in a previous paper. The MVE:M, M = Ar, N, system affords one of the very few examples for conversion by i.r. radiation of conformers of a molecule which features no internal hydrogen bond: other examples are CHzFCHzF [ 151 and CHSCH2CH20H.* These molecules contradict the proposition made occasiooally [ 141, that H-bond forming groups were a necessary structural element of any molecule to show conformer interconversion by i.r. irradiation. Furthermore MVE:M might serve as an example for the potential of combined TMB- and i.r.-interconversion of conformers for the nearly complete determination of the vibrational spectrum of a rare conformer. From gas phase spectra the determination of the vibrational spectra and of the structure of the unstable conformer so far proved possible to a very fragmentary extent only.

2. EXPERIMENTAL 2.1. Instrumentation Thermal molecular beam experiments were made with a equipped with a liquid helium (LHe) bath cryostat of our own design, operating in transmission mode (CsI window). Matrix gases were introduced through nozzles arranged symmetrically on the outer wall of the cooling jacket surrounding concentrically the Knudsen cell. The latter (15 mm diam. x 18 mm length cylindrical cavity) was made from stainless steel 316 with the heater (Thermocoax, Philips Comp., Ziirich, 1.0 mm diam.) vacuum soldered directly onto the cylinder; in order to

Perk&Elmer Model 325 Spectrophotometer

l Private communication by Professor J. MURTO prior to publication. WCLI type L-06609. *Edwards, Ltd., Diffstack 100/300. &&e Shore Crvotronics, Inc., Si diode DT-500 and temperature controier DTG 500. llOCL1 type LO6743-9. TCoherent Radiation Laboratories, Model No. 201.

reduce thermal decomposition the cell may be operated with a boron nitride cylindrical insert. Temperature was measured in the free space of the cavity of the Knudsen cell by the aid of a 0.5 mm diam. iron constantan thermocouple (Thermocoax) referenced to a triplet point tube. Methylvinylether (MVE) was introduced from a 21. gaseous sample of 8-16 Torr pressure through the Knudsen cell operating in flow mode. Usually a Knudsen orifice of 0.5 mm diam. was used. This results in a residence time of 10-20 ms in the hot space of the Knudsen cell. The cryostat was mounted on top of a prismatic vacuum chamber inserted into the sample compartment of the spectrometer, such that both reference and sample beam pass the vacuum chamber through CsI windows. The space between the vacuum chamber and illumination compartment of the spectrophotometer allowed mounting of i.r. interference. filters or wire meshes in either sample or/and reference beam. Infrared band pass filterst with band pass 145&6OOcn~’ were used for TMB spectra in order to suppress photoconversion of MVE during measurements of spectra in the band pass region. Infrared irradiation experiments were carried out with a LHe bath cryostat of our own design, operating in the reflection mode mounted in the sample compartment of a Perkin-Elmer Model 225 spectrophotometer. The cryostat is mounted on top of a prismatic vacuum vessel directly connected via a LN2 trap to a Diffstack diffusion pump;* both sample and reference beam pass through the vacuum compartment through CsI windows, in order to keep the two beam geometries as equal as possible. A goId-plated copper mirror forming part of the bottom of the LHe dewar serves as a target. For temperature control not far from LHe temperature a thermocoax heater mounted on a copper cylinder between mirror and bottom was used. For temperature measurement either Au/Fe-Chrome1 or Cuconstantan thermocouples referenced to the triple point of Hz0 or a silicon diode connected to a temperature controller§ mounted on the back of the Cu mirror were used. By this arrangement matrix temperatures near LHe temperature were reached (typically 5 K as measured by the diode). The mirror and bottom of the LHe dewar were enclosed in a gold plated copper shield kept at LN, temperature, equipped with slots for admission of the molecular beam and the i.r. sample beam. In order to avoid formation of ice films on the mirror before deposition of the matrix, the LHe dewar was rotated such that the mirror face opposed the LNI shield. By this procedure formation of icefilm could be greatly reduced. The optical arrangement used in this work is similar to a set-up used earlier for i.r. photcconversion by CO, laser radiation [ 161 with the laser replaced by an external globar. Typically the reflection system showed transmission of the order of 70 %. It should be mentioned that by this arrangement the matrix i.r. spectra may be continuously measured during external irradiation experiments. In order to avoid photoconversion during deposition and external irradiation, the radiation of the instruments globar was bandpassed by interference filters.I( For irradiation with appropriately band passed i.r. radiation, the radiation from the external globar (diam. 6 mm) fed by a stabilized power supply is imaged by a quartz lens doublet (1, = 220 mm, D, = 85 mm, _fz= 200 mm, D2 = 85 mm) through BaF, or quartz windows onto the target mirror, such that the image of the spectrometer’s globar coincided with the image of the external globar. The radiation power of the latter falling on the mirror is bandpassed according to the transmission of quartz (see below), the incident power was calibrated by means of a laser power meter1 as a function of electrical power supplied to the globar. As a consequence of the oblique incidence of the sample beam onto the matrix layer in the reflection setup the effective layer thickness d,, z 2.41 dmatnx. In matrix preparations the deposition rate and layer thickness (dmatex, dmt& were measured by the aid of

Infrared spectroscopy of MVE

321

interference fringes generated by a small HeNe laser [17]. Hole burning experiments by the CO, laser were carried out with a set-up reported earlier [16]. The following list gives a view of the radiation power reaching the target mirror in i.r. irradiation experiments.

Source

Band pass

Power

Globar 225 Ext. globar Globar 225 CO, laser*

Broad band 330&27OOcn-’ OCLI LO6743-9 R(38)

1.22 w 4601230mW 30 mW ,<2w

*Apollo Laser model 55A. 2.2. Chemicals For matrix gases Ar, purity 0.999999 and Nt, purity 0.999998 (both Messer-Griesheim) were used and deposited steel inlet through precision stainless systems. Methylvinylether, purity 0.99, was purchased from MesserGriesheim and was used after trap to trap condensation to the storage bulb without further purification. Pressure of MVE in the latter was monitored by a Baratron (100 Torr range).* 2.3. Treatment of data Since no automatic data acquisition system for digital measurement of spectra was available for this work, the records of the spectra were digitized by the aid of a digital plotterdesk computer system; usually 3 10 points per (observed) half width at half maximum (HWHM) were measured. The data (10%1600 points per bandgroup) were then treated by the aid of a program package [ 181 allowing calculation of linear or quadratic baseline, optical density and integrated absorption. Much of the data were subject to line shape analysis (LSA) by the aid of an interactive line shape program written for a desk computer) [ 181, approximating observed band contours by overlapping Lorentzian bands. From the LSA data approximate true line parameters were then derived by application of the LSA based deconvolution procedure described in a recent paper [19]. Since direct determination of the analytical concentration of MVE in the matrix was not possible, only integrated absorption (jband(- In T(it))di) will be available from the spectral data. However for some bands estimates of integrated absorption coefficients could be derived. By rather general experience LSA procedures should be used critically and data derived therefrom considered with care. A few remarks concerning various aspects of LSA results will be made in Section 5, furthermore the reader is referred to our recent paper [19] and the references given therein. 3. RESULTS In Fig. 1 the effective transmission of the quartz optics used for imaging the external globar onto the matrix deposited on the target mirror is shown, together with the spectral radiation power directed toward the target for both 460 and 230 mW total power. Thesecurves will serve to estimate the radiation power effectively transferred to the MVE:M system. Typical spectra of MVE: M, M = Ar, Nr are reproduced in Fig. 2, taken with the reflection set-up. Furthermore Fig. 2A has been used to derive the

*MKE Instruments, Inc. t HP 9826.

3 0

cmFig. 1. Spectral characterization of NIR radiation for photoconversion. Solid line: filter transmission; -. -. -- and . -. -. .-: spectral radiation power P(3) (erg s- ’ cm) at 460 mW and 230 mW total power, respectively; - -- - ~ and --: effective spectral power I,,(S) (erg s-i cm) according to the Kubelka-Munk model for scattering matrix at 460 and 230 mW total power, respectively.

scattering function of Ar matrices deposited at LHe temperature on the target mirror. In order to obtain approximate true transmission values for spectral bands, this function may serve as an overall base line. As a rule scattering of the matrix may vary widely over the spectral range 4OOC200 cm- i; therefore base line transmission has kept piecewise near 0.8-0.95 by the aid of a calibrated attenuator in the reference beam. The scattering function has been determined by least squares according to the expression (d,,: effective layer thickness)

a(P) =

c atck. k

Equation (1) originates from the familiar Kubelka-Munk description of light propagation in scattering media, provided absorption is vanishingly small [20]. Typical scattering coefficients as found from the spectrum in Fig. 2A by least squares analysis are as follows (Tbl(0) z 0.625) u2 z 2.3443 x 1Om6cm 03 z 7 .0036 x lo-”

cm’.

The values show the scattering process to be of a mixed Mie type; obviously the inert gas crystal fragments formed under the depositionconditions used in this work vary over a considerable‘range in size. The spectra shown in Fig. 2 were taken after approximately 6.5 h irradiation with the external

322

TIMOTHY BEECH et al.

Infrared spectroscopy of MVE globar and subsequent 1 h (approx.) irradiation with the instrument’s globar. Further experimental conditions are given with the figure. Attempts to produce hole burning by CO1 laser irradiation are documented by Fig. 3 for the v,,ay?(CHJ) fundamental of the (unstable) gauche conformer. In this figure a measurement of the slit function by means of the R(38) laser line (1088.97 cm- ‘) is included. Analysis of the signal produced by the laser emission, assuming triangular slit function yields a spectral slit width of 0.45 cm-’ full width at half maximum (FWHM), whereas from the instrument’s calibration, 2s = 0.43 cm-’ (FWHM) is predicted. In Fig. 4 sections of MVE:Ar spectra generated by thermal molecular beams (TMB) with Knudsen cell temperatures between 300-950 K are reproduced. The components obtained by lineshape analysis of particular bands are shown. For TMB spectra the deposition conditions were not systematically kept constant, however in Table 1 a typical example for deposition conditions is included. The figure comprises a number of bands between 1250 and 930 cm- ‘, which will serve to discriminate conformers and sites, to demonstrate the dependence of line contours on Knudsen cell temperature and to determine the conformer conversion enthalpy. Typical line parameters characterizing the bands of Fig. 4 are collected in Table 2 and Table 3. Deposition conditions for NIR irradiation experiments with nominal radiation power 460 and 230 mW respectively are collected in Table 1. The temporal behaviour of important bands in the frequency region 1400-800 cm- ’ is demonstrated by Figs 5 and 6. The former illustrates for MVE:Ar the evolution of contours of bands

323

Table 1. Matrix preparation conditions is

hIR*

Exp.

M

(mW

NIR

Ar Ar N, N, Ar

460 230 460 230 -

1 2 3 4 TMB 15

(rm/min) 134 137 145 205 66

1.85 1.90 1.95 2.92 0.9111

*Power incident onto matrix; for spectral composition see Fig. 1. tEffective layer thickness for i.r. spectroscopy (oblique incidence in reflection cryostate). %OS z 4.5 K, deposition rate of matrix layer. $Typical matrix deposition condition for TMB experiments. 11 Tdcporz 11K.

increasing (1238, 1136, 1089 cm-‘) or decreasing (1216, 894, 812cm-‘) in intensity as a function of duration of irradiation, both for 460 and 230 mW experiments. By contrasting in Fig. 5 band contours for particular vibrational transitions observed with 460 and 230 mW irradiation power and approximately the same fluence (irradiation time x irradiation power), the conformer conversion is shown to be a onephoton process. Furthermore, from such plots integrated absorption intensities have been determined, which in turn were used to construct the growth curves discussed in Section 4. Part of the temporal behavior of the bandcomplexat 1321/1311 cm-’ of MVE:Ar and MVE:N, is documented by Fig. 6; in this graph also are included the results of line shape analysis. Line

Fig. 3. MVE:Ar: hole burning by R38 CO2 laser line incontour of gv,.+yi(CHs) near 1089 cm-’ (cf. Table 8). M/A ‘v 1500, LHe temperature, laser power PO z 1,6 W, profile of k38 shown for determination of spectral slit width (nominal 0.43 cm-’ FWHM).

324

TIMOTHY BEECH et al.

-6 II

0

372K

438K

Fig. 4. MVE:Ar: band contours of thermal molecular beam (TMB) spectra. M/A z lOOC1600, deposition conditions see Table 1, Knudsen cell temperatures respectively 372, 438, 571, 922 K, results of line shape analysis cf. Table 2.

parameters derived by the latter are collected in Table 4. The data should demonstrate: (i) the band complexes at 1321 and 1311 cm-i originate from the stable and unstable conformer, respectively. (ii) under the deposition conditions used in this work band contours of MVE:N, are much more complex than those of the MVE:Ar. (iii) in spite of widely different band contours, integrated intensities are similar for both systems. Figure 7 shows the transformation of the contour of

bands at 827 cm- ’ (unstable conformer) and at 8 17/812 cm- ’ (stable conformer) during the photoconversion of the stable to the unstable conformer of MVE:Ar by NIR irradiation (nominal 46OmW). The contours were subject to LSA; the resulting components are indicated in the figure and the line parameters are collected in Table 5. These data illustrate the highly complex character of the photoconversion process by the fact that the site structure changes with duration of NIR irradiation. Bands of the different conformers generated by

Infrared spectroscopy of MVE Table 2. MVE:Ar

line parameters of cis and gauche

325

conformer in TMB spectra

372K*

438 K

571 K

922 K

372 K

438 K

571K

922 K

1239.5t 1.70 0.53/0.46$ 1238.6 0.90 0.59fO.52 1237.1 0.32 1.19/1.15 1218.7 2.15 1.70/1.63 1216.87 5.57 0.42JO.32 1215.4 0.65 0.3310.22 1213.5 1.35 1.13/1.03 1208.0 0.80 4.7 1200.3 0.71 0.9210.87 1194.6 0.25 0.70/0.64

1239.4 0.75 0.58/0.51 1238.5 0.74 0.5010.43 1237.0 0.20 0.95/0.90 1218.7 1.64 1.92/1.82 1216.9 3.06 0.39/0.28 1215.7 0.52 O&i/O.38 1213.7 0.80 1.12/1.07 1207.7 0.67 6.07 1200.4 0.39 0.83/0.78 1194.7 0.17 0.7810.72

1239.4 1.08 0.55 1238.5 0.93 0.46 1237.1 0.49 1.16 1218.7 1.33 1.75 1216.9 3.10 0.38 1215.6 0.41 0.38 1213.7 0.78 1.27 1209.2 1.33 7.48 1200.5 0.34 0.79 1194.7 0.12 0.59

1239.4 1.57 0.46 1238.5 2.31 0.60 1236.9 0.76 1.16 1218.8 1.27 1.81 1216.8 3.35 0.42 1215.5 0.32 0.28 1213.7 0.74 1.10 1209.8 1.37 7.09 1200.4 0.44 0.89 1194.5 0.13 0.54

1136.1 0.98 0.30 1135.2 1.05 0.56 1133.7 0.12 0.79

1136.0 0.98 0.33 1135.2 0.82 0.48 1133.7 0.15 0.78

1136.2 1.41 0.3 1 1135.2 1.28 0.56 1133.6 0.22 0.82 1129.6 0.11 1.06 1090.8 0.29 1.19 1089.3 0.35 0.44 1087.5 0.24 0.56 1012.1 0.01 0.25 1011.2 0.23 0.42 1009.4 0.26 0.92 1008.0 0.70 0.27

1136.1 2.41 0.29 1135.2 2.70 0.56 1133.6 0.35 0.73

1004.6 0.05 0.78

1004.5 0.03 0.68

1090.7 0.04 0.57 1089.3 0.51 0.78 1087.4 0.16 0.50 1012.0 0.04 0.22 1011.2 0.36 0.41 1009.2 0.59 1.08 1008.0 0.64 0.18 1007.8 0.49 0.25

1090.6 0.10 1.08 1089.4 0.36 0.63 1087.4 0.20 0.70 1012.0 0.05 0.27 1011.3 0.19 0.38 1009.3 0.50 1.34 1008.1 0.55 0.23 1007.9 0.11 0.20

1090.7 1.07 1.75 1089.0 0.65 0.47 1087.6 0.33 0.41 1012.0 0.03 0.26 1011.3 0.26 0.48 1009.4 0.25 0.85 1008.1 0.75 0.27

*Knudsen cell temperature TKc. tThe three entries denote respectively: resonance frequency 8,, integrated absorption ALSAand line width yLSA(HWHM) in cm- 1derived by line shape analysis (LSA). As a rule only components with > 5 o/oof total integrated absorption are included in this table. *Second entry of the line width parameter denotes approximate true value obtained from deconvolution based on LSA.

Table 3. MVE:Ar: integrated absorption of bands of TMB spectra ‘KC

(K)

1238

1216

1136

1089

372

1.89* 1.93t 2.011 1.57 1.62 1.68 2.28 2.36 2.45

10.95 10.97 11.52 6.90 6.90 7.33 6.92 6.93 7.51

2.04 2.11 2.18 1.91 1.96 2.02 2.83 2.92 3.02

0.62 0.66 0.68 0.65 0.66 0.70 0.85 0.84 0.88

438

571

1.97 1.97 2.09 1.27 1.28 1.40 1.14 1.16 1.25

*First entry: integrated absorption (cm-‘) obtained by numeric integration of experimental optical density Jt,r,ndDexp(i+dJ(no wing correction). tSecond entry: integrated absorption (cm-‘) by numeric integration of LSA optical density ~bendDLSA(J)dc(no wing correction). $-Third entry: sum of integrated absorption of components of band contour determined by LSA.

326

TIMOTHY

BEECH et al.

00

LI 24

rnln

45

ml”

163 324

,\

0

30 54

OD

57 34

32 157

107 212

139 278

172 336

.3

i

‘.

0~ 1033

crn~c-I>

.

: 1073

146

238

180

liii 356

A!!!_ Fig. 5. MVE:Ar:

temporal

behavior

of matrix spectra under NIR irradiation. Table 1 (M/A % 1400).

For deposition

conditions

see

Infrared spectroscopy of MVE

22min

L

39mi

0min

n

A..,

231min

327

3min

1 274min

298mi

335mln

n

. 73

.7!

.I

. 3

. 93

.a!

e

I!

12

1

1316

13

Fig. 6. MVE: M: M = Ar, N1; temporal behavior of 1321/1311 cm-‘-band complex under NIR irradiation. M/A 21 1400, P, z 460 mW, LHe temperature.

268min

822

cm^(-1)

006

Fig. 7. MVE:Ar: temporal behavior of band contours (M/A x 1400, LHe temperature, PO z 460 mW). 827cm-’ gv,,a. 817/812cm-’ cvlla”.

TIMOTHY BEECH et

328 Table 4. MVE:M

al.

(M = Ar, NJ line parameters of 1321/1311 cm-’

Ar* r$ (min)

3,s (cm-‘)

21.58

A (cm’)

Y (cm-‘)

rS (min)

1327.5 1326.7 1323.2 1321.0 1311.8

0.10 0.12 0.21 1.30 0.10

0.52 0.66 1.09 0.50 0.65

0.0

39.00

1327.7 1326.7 1322.3 1320.9 1311.2

0.07 0.12 0.07 1.15 0.12

0.52 0.73 0.38 0.49 1.04

231.33

1326.9 1320.9 1315.0 1312.7 1311.4

0.14 0.55 0.09 0.10 0.16

0.10 0.50 0.66 0.92 0.55

273.83

1327.1 1320.9 1315.3 1314.4 1311.3

0.18 0.57 0.06 0.09 0.25

0.84 0.51 0.44 0.60 0.85

band contour N,t

“, (cm’)

A (cm-‘)

Y (cm-‘)

1324.7 1323.8 1322.5 1320.8 1319.2 1312.4 1311.3

0.31 0.85 0.44 0.16 0.11 0.02 0.03

0.40 0.64 0.91 0.88 0.84 0.49 0.57

3.20

1324.5 1323.6 1322.2 1320.5 1311.7

0.44 0.88 0.32 0.34 0.09

0.54 0.70 0.99 1.49 1.12

287.63

1325.1 1324.2 1323.2 1321.9 1312.6 1311.6

0.31 0.49 0.21 0.06 0.11 0.11 0.12

0.53 0.66 0.80 0.70 0.86 0.61 0.77

1324.8 1324.1 1323.2 1321.7 1320.1 1310.1 1309.1

0.16 0.37 0.50 0.15 0.20 0.09 0.32

0.43 0.56 0.75 1.00 1.63 0.42 1.02

1320.9

334.82

*M/A z 1400, deposition conditions see Table 1, experiment NIR 1. TM/A z 1350, deposition conditions see Table 1, experiment NIR 3. flrradiation time, nominal irradiation Rower 460 mW. OF,, A, y denote resonance frequency, integrated absorption and line width parameter (HWHM) as determined by LSA (not deconvoluted).

either TMBs or by NIR irradiation were found to differ in some cases drastically. This is illustrated by Fig. 8 and Table 6 for the band complex near 965-940 cm-‘. The TMB and NIR conversion spectra are given for four Knudsen cell temperatures and for two short and two long irradiation times, respectively; the corresponding results of the LSA of the TMB spectra presented in Table 6 will serve to analyse the relation between the 953 and 943 cm- 1band contours. Both belong to the g conformer and also are included in the determination of the conformer conversion enthalpy. Similar phenomena have been observed with the system MVE:N,; in this case the band contours are even more complex and more difficult to analyse. Hence this system will not be discussed in greater detail in the present paper. 4. ANALYSIS AND DISCUSSION The results presented above will be analysed in the following order (i) thermal molecular beam (TMB) spectra, (ii) photoeonversion of conformations of MVE by i.r. irradiation, (iii) assignment of i.r. spectra of conformers.

Subsequently it will be assumed that (spectroseopitally) one further conformer besides the s-cis (c) conformer exists and it will be denoted by s-gauche (g). 4.1. TMB spectra and enthalpy of conformer conversion 4.1.1. Spectra. First a number of comments relating to the spectra shown in Figs 4,5,6 and documented by Tables 2, 3, 6 will be made. From either set of information it is clearly seen that the integrated absorptions of bands at 1238, 1136, 1089, 953 (943) cm-’ increase, whereas bands at 1216,1008 and 963 cm- ’ decrease, with increasing TMB temperature (TKJ. Though not documented in detail by TMB spectra, this should be completed by stating that bands at 1321 and 1156 cm-’ decrease, whereas the weak band at 1311 cm-’ increases with increase of TKc. Hence the unstable conformer(s) is (are) trapped from the TMB; growing bands uniquely characterize less stable conformers and conversely. All bands studied by TMB experiments possess complicated contours. This is best illustrated by the results of line shape analysis (LSA) shown both in Figs 4 and 5 and Table 2. In the latter only components with integrated absorption 2 5 % of the total integrated absorption of a band contour are listed. Each of the bands in the frequency

Table 5. MVE:Ardependenceof line parametersof complexband contourson timeof NIR irradiation* Gauche

Cis v,,a”y,(:CHz) 817cm-’ contour

vzIa”y,(:CH,) 827cm-’ contour7

r1 (min) (cm-‘) 17.58 (0.78)8 42.50 (1.21) 92.50 (1.65) 184.53 (2.21) 224.17 (2.24) 265.45 (2.66) 318.28 (3.05)

t

(cl&

(cn&

0.11 0.12 0.29 0.22 0.10 0.11 0.27 0.35 0.23 1.30 0.33 0.33 0.31 0.34 0.44 0.15 0.44 0.11

(US)

0.71 0.48 0.59 0.45 0.36 0.42 0.75 0.60 0.67 0.64 0.47 0.64 0.30 0.48 1.01 0.32 0.86 0.44

(CAl)

828.6 826.8 825.0 828.5 826.4 824.6 829.2 825.8 828.9 826.9 824.9 828.6 826.8 825.2 828.5 825.4 828.7 825.4

(I&)

(ctS)

(min) (cm-‘)

0.09 0.08 0.09 0.51 0.20

0.58 0.42 0.58 0.65 0.46

827.4 825.9 823.7 827.1 825.3

1.02

0.83

827.5

0.23 0.11

0.63 0.47

827.7 825.7

94.33 (2.47) 186.12 (1.81)

1.01 0.22

0.54 0.53

827.3 826.1

226.17 (1.66)

1.77 0.26 2.00 0.49

0.61 0.36 0.60 0.58

826.9 824.9 827.8 824.9

268.45 (1.43)

(I&

(cn&

(c&)

18.33 (3.39)

0.11 1.65

0.49 0.74

818.8 812.5

44.67 (3.11)

0.11 0.46 1.37 0.12 0.99 0.11 0.72

0.33 0.79 0.49 0.43 0.47 0.41 0.49

818.7 813.0 811.7 818.9 812.2 819.1 812.3

0.08 0.12 0.75 0.11 0.51

0.26 0.64 0.47 0.39 0.45

819.3 813.9 811.8 819.1 811.7

(c&

(c&)

(CtS)

0.14 1.49

0.92 0.49

817.7 811.9

1.09

0.48

812.3

0.38 0.89 0.12 0.82

0.68 0.45 0.45 0.47

813.0 811.7 813.4 811.8

0.07 0.57

0.38 0.57

818.8 812.4

0.74

0.62

812.2

*M/Az 1400,for depositionconditioncf. Table 1,experimentNIR 1. ?A,y,c,denoteintegratedabsorption,linewidthparameter(HWHM)andresonancefrequencydeterminedby LSA,respectively(cf.Ref.[ 193).Componentswith lessthan 5 % integratedabsorptionof total integratedabsorptionare omitted SIrradiationtime,nominalirradiationpower460mW. &urn of integratedabsorption5of all componentscomposingthe contour.

.. - - -

-_1

-

c..

c

-.

.~.

-

_

-

.I

.”

._

-

_

_

_

.”

,

330

TIMOTHY

BEECH

et al.

(b)

OD 372K

F P I lmln

2s

m

OD

436K

35mln

62 2

25

OD

3 571K

54mln

25

Oil 922K

34am1n

25

rL_ 170

cm*(-I)

960

950

940

972

cm-C-1

1

960

957

949

947

939

Fig. 8. MVE:Ar: TMB and NIR irradiation band contours of conformers (M/A z 1200, LHe temperature). 963cm-‘; cv,,a”; 953 cm-‘, 943cm-‘: gv,+. (a) TMB band contour. (b) Band contours generated by NIR irradiation. range 1350400 cm- ’ consists of at least three significant components.

The following comments apply: (i) integrated absorption obtained from numeriof observed optical density cal integration (j,_,D(S)dS) and sum of integrated absorption of all LSA components agree within 2-5 %, the former being systematically lower owing to truncation of the integration domain (wing correction) and systematic errors of the LSA (cf. Section 5 and [19]). (ii) for stronger components of the 1238 and 1216cm-’ contours approximate true line width parameters (HWHM) are given in Table 2. The data illustrate the necessity to correct the apparent line width parameter YLsa b y deconvolution, since it deviates signticantly from true line width for narrow component bands. (iii) the 1238 cm- 1 band (g conformer) consists of three significant components; the lowest frequency component featuring yLSAZ 1 cm-‘, i.e. much larger line width than the two high frequency components (Yz YLSAz 0.4-0.5 cm-‘). All three components ex-

hibit similar dependence of A,,, on TKc, and no significant increase of y with TKc seems detectable. (iv) the 1216 band complex (cis conformer)consists of one strong band centering at 1216 cm-’ and at least two satellites (1200,1195 cm- ‘). The intensity ratio of the latter varies only slightly with TKc, hence one is tempted to classify the satellites as site of the 1216cm-’ transition. These two bands cannot be excluded with certainty to be related to vibrational transitions different from the 1216 cm-’ fundamental. The strong component itself decomposes into at least five components. One of them is unusually broad and is implied mainly by the fact that optical density between the main maximum and the satellites remains positive. Also on the high frequency flank of the main maximum a broad component (yLSAz 1.7 cm- ‘) is located by LSA. The component with highest in1216 cm-’ tegrated absorption at features yLsAz 0.30 cm- ‘, tending to increase with increasing T,c.Thecomponentat 12156(l) cm-’ mayserveasan example for large systematic errors associated with weak bands located in the lobes of strong bands: such

Infrared spectroscopy of MVE Table 6. MVE:Ar line parameters of 963-953-943 cm-’ band contour (TMB-spectra) 438 K

TKC

571 K

922 K

967.30*t 0.04 0.70 964.75 0.67 1.16 963.01 0.58 0.45 962.51 0.13 0.34 952.58 0.36 1.06 951.86 0.03 0.32 949.34

968.71 0.03 1.22 965.22 0.26 0.80 963.87 0.16 0.60 963.05 0.41 0.46 953.30 0.05 0.34 952.56 0.24 0.88 950.04

968.06 0.04 1.50 964.82 0.34 1.02 963.35 0.15 0.42 962.89 0.26 0.35 952.98 0.23 0.66 952.18 0.16 0.53 949.44

964.61 0.43 1.19 962.93 0.31 0.43 962.46 0.16 0.30 953.06 0.19 0.54 952.17 0.60 0.73 949.00

0.06

0.11 2.20 945.41 0.13 2.89 942.18 0.15 2.66

0.12 1.95 944.45 0.20 2.34 941.42 0.12 1.57

0.22 1.90 944.62 0.19 2.09 941.89 0.19 1.34

1.97 943.88 0.31 3.32 940.99 0.04 2.06

*Components of band contour. tFirst, second and third entry respectively denote resonance frequency i,, integrated absorption A,,6 and line width oarameter Y(HWHMI of Lorentzian comoonents of the band contour bekeen 980kd 935 cm- ‘, as deiermined by LSA, cf. Ref. [19].

bands are usually not well determined by LSA. Finally the LSA data indicate a systematic variation with TKc of the relative intensity of some of the components; e.g. the pair 1218.7, 1216.8 cm-‘. (v) the band contour near 1136 cm-’ (gconformer) consists of at least three significant components whose integrated intensity ratios essentially remain the same in the whole TKc range studied, though the line width parameters differ by at least a factor of 3. (vi) the band complex at 1089 cm-’ belongs to the g-conformer and is composed of three components, which fairly distinctly change relative integrated ab. . sorption with mcreasingTKc. At the same time the high frequency component features increasing line width and the middle component shows falling line width with increasing Knudsen cell temperature. (vii) the 1008 cm- ’ band complex belonging to the cii conformer is built from four to six components of widely varying line width (0.25-l cm-‘), integrated intensity (20: 1) and strongly different dependence on T KC’ A further instructive example for dependence of line contours in TMB spectra on Knudsen cell temperature is provided by the band system at 963953943cm-’

331

(Fig. 8). In this example comparison of line contours observed with TMBs and with NIR irradiation experiments will prove important, hence these bands will be discussed in connection with the latter. 4.2.2. Enthalpy of conformer interconversion [21]. Symbolizing the conformer interconversion process by the stoichiometric formula -c+g*=o where g + and g _ symbolize the two isometric forms of the gauche conformer (internal rotation angle v(C*O-C:C) = 0 for cis and v(C-0-C:C) = f v@ v, # 0, f z for gauche), thermal equilibrium of the process is described (to a first approximation) by the expression ln (x,+/x,) = ln (x,44 +

where

= -

W”(T,YW

@W-d/R),

x0+ = xg_: = &,

xg++xg_ +x, = I,

(2)

In (x,/x,): = In K(T) = - (AH” (T,)/RT) - In) + WV’o)IR). If the TMB emerging from the Knudsen cell is suddenly frozen in the matrix, the concentration ratio co/cc of the conformers in the matrix (at high M/A) equals xJx, in the Knudsen cell and is determined by the ratio of integrated absorption A = E . c . d,, of any pair of bands belonging to the gauche and cis conformer (for any value TKc) x,/xc = c&z, = A (9:; T,&A(P;; TKc).

(3)

In order to keep errors small it is advantageous to use in Eqn. (3) integrated absorptions of pairs of adjacent bands measured with similar spectral slit width or to use approximate true integrated absorption derived from deconvolution based on LSA [19]. According to Eqn. (2) plots of ratios A(P;T)/A(SE;T) vs T$ should yield parallel lines with slope - AH”(7’,)/R; the ordinates of such lines however are determined up to a constant only. In Table 3 integrated absorptions A($; TKt-) of the main TMB bands between 1300 and lOOOcm_’ determined for four Knudsen cell temperatures TKc are collected for each TKCvalue. In order to demonstrate the spread of A($; TKc) values when determined by various methods, there are three values given for each temperature, namely JbPnaDexp(V)dP(numerical integration of experimental optical density over band area), jbpndD,,, (i+iV (numerical integration over band area of optical density fitted by LSA) and c AiLsa (sum of integrated absorptions of components determined by LSA of the experimental band contour). In most cases, as is to be expected s_D,,,(J)d+

G JbdDLsA(g)dt

< CAiLsA.

Furthermore, if LSA data are subject to deconvolution [ 193 in most cases CAiLsa ,< =&4:;zoN,

332

TIMOTHY BEECHES al.

but the two values are frequently equal within statistical errors; therefore no approximate true integrated absorption values are listed in Table 3. Since the band complex 963-953-943 cm-’ also comprises both cis and gauche bands, line parameters of these are collected in Tables 6, 7 and also were used in determination of AH”. Actual calculation of this quantity may be based on various pairings of bands. These are indicated in the Van? Hoff plots of Fig. 9. From least squares analysis of the latter the value AHo

z 6.62(35) kJ/mol x 1590(80) cal/mol

is obtained. This value is significantly higher than the value AH x 1150(250)cal/mol reported by OWEN and SHEPPARD [22], which subsequently has been used by DURIG and COMPTON for determination of the potential associated with internal rotation about the C.O-C:C bond [23]. It is however lower than a value predicted by quantum chemical calculation on the MP 2/6-3 1G level, which yielded EgoUhe - Ecis z 8-9 kJ/mol[24]. Regarding the vibrational spectra of the two conformers found in this work E, - EC is expected to deviate but little from AH”(O) and AH”(T,). The most important contribution to the deviation originates from widely differing barriers to methyl group rotation. There arises the question whether thermodynamical arguments would allow one to identify the unstable conformer as having either gauche or tram r, structure. This would require complete determination of the right hand side (rhs) of Eqn. (2) not only up to a constant, since in case of a planar skeletal r,-structure of the second conformer, the term - Ini is lacking. In turn this implies direct determination of the analytical concentration of the two conformers for at least one temperature TKc. In the course of this work direct analytical concentration measurements have not been possible. Recently an example for determination of the entropy AS” of a t -+ g conversion of a matrix isolated molecule (CH2FCH2F) has been reported [15], which indicates that contributions from the matrix cannot n priori be neglected. For MVE statistical equilibrium

Table 7. MVE:Ar

integrated

absorptions

-3

333

K

Fig. 9. MVE: M Van? Hoff-plot for c-is-gauche equilibrium. Plot of ratios of integrated absorption of band pairs vs reciprocal Knudsen cell temperature. ~ Experimental data. - - - Least square lines. Band pair (cm-‘): 1238/1216 (I), 1136/1216 (2), 1136/1008 (3), 1216/1008 (4), 953/963 (5), 953 941/963 (6), 953 . 949/963 (7).

calculations possibly are useful in the elucidation the r, structure of the second conformer.

of

4.2. Conformer interconversion by NIR radiation First some statements should be made regarding the radiation used for photo conversion. Figure 2 shows the spectral radiation power density to peak near 31OOcm-‘, to be very low between 3400 and

of 963-953-943 spectra

At (953-952)

500

1000

cm-’

band contour

ofTMB

T

A* (963)

A$ (949-941)

A$(953-941)

(g

(cm-‘)

(cm- ‘)

(cm-‘)

(cm-‘)

312 438 571 922

1.42 0.86 0.79 0.90

0.39 0.29 0.39 0.79

0.40 0.38 0.43 0.60

0.80 0.66 0.83

1.39

*A(963) integrated absorption of band contour centered at 963 cm-’ derived from LSA; for components of the contour cf. Table 6. tA(959-952) integrated absorption of band localized near 953 cm-’ (for LSA cf. Table 6). $A(949-941) sum of integrated absorption of band contour localized in frequency range 950-940 cm ‘. §A(953-941) sum of integrated absorption of all components of band contour localized in frequency range 953-940 cm-’ (cf. Table 6).

Infrared spectroscopy of MVE 3750 cm- ’ and to follow the Planck radiation function above 4200 cm-‘; the two curves for 460 and 230 mW being nearly proportional in spite of different globar temperatures. Owing to scattering the spectral density in the matrix differs considerably from the incident spectral density, with regard to both magnitude and shape. This is manifested by the curves for I,,(3) = P(P). T,,(5) in Fig. 1; in particular the short wavelength wing is considerably attenuated by the scattering process. It is obvious that determination of quantum yields for photoconversion requires one to take matrix scattering into account. Since the latter alters spectral radiation power density in a linear manner conversion rates should still be proportional to incident power density. Before discussing interpretation of experimental findings, a few remarks concerning the theoretical background should be made. The free nonrigid MVE molecule possesses the isometric group X(r, u) = g3 (dihedral group of index 3) is identical with the isometric group of the associated semirigid model. The latter features a rigid frame with C, local symmetry and two tops with C,, and C, local symmetry, respectively (C,F Cj,TC,T’ model)[25, 26, 271. The nonrigid model may be described by two finite and 22 infinitesimal degrees of freedom (internal rotation angles 7 and u for CHs and CH:CH1 groups and conventional small amplitude internal coordinates). Internal rotation-vibration states are to be classified as r(‘+), Fco-) and F(l) (A,, AZ, E). Infinitesimal modes oflocalized cis, tram and gnuche conformers are to be classified according to the covering groups C,, C, and Ci respectively. It will be shown in a forthcoming paper that each type of internal rotational state is coupled to at least one symmetry type of the infinitesimal modes; i.e. excitation of a vibrational mode will always (in principle) be followed by energy transfer to internal rotation states. The increase (decrease) of optical density of MVE: M bands induced by NIR irradiation, as documented by Figs 5, 6, 7, 8 give rise to the following questions: (i) does photoconversion involve the same unstable conformers as found from TMB experiments? (ii) is the photoconversion process a one- or a twophoton process? (iii) can one extract information about the barrier to conversion of the conformers? With regard to the first question, the experimental data available show both type of experiments to involve the same conformers. It however should be kept in mind, that they may produce different types of substitution of MVE into the matrix crystal, as exemplified by Fig. 8. Regarding the mechanism of the NIR photoconversion process, the following statements first should be made: - radiation in the frequency range 1400-700cm-1 does not induce c -+ g conversion of conformers at a measurable rate. If conversion were produced by

333

radiation of this energy, the rate would be at least 10 times lower than produced by the NIR radiation available, but still would be unambiguously measurable. - determination of conversion quantum yield by NIR radiation is severely influenced by the scattering of the matrices, which in turn depends on deposition conditions. Since the latter were chosen to keep thermal relaxation as low as possible, irradiation experiments were conceived to show the conversion process to be a one-photon process, independently of accurate quantum yield measurements. Within the Kubelka-Munk model for absorption in scattering media, the extent of conversion should still be proportional to irradiation time x radiation power (fluence), provided the conversion process is one photon. In Fig. 5 band contours of three growing and three falling bands observed during irradiation with PO = 460 mW and PO = 230 mW, respectively, are superimposed at times corresponding to approximately equal fluence. In most cases superimposed contours match as closely as can be expected, showing the photoconversion to be a one-photon process. This same result is obtained for all bands of the two conformers in the region 1400-800 cm- ‘. The band complex near 963-953-943 cm- ’ deserves some specific remarks. As shown in Fig. 8 (parameterized in Tables 6 and 7) TMB and NIR conversion experiments yield widely different contours of the 953-943 cm- ’ band complex, whereas similar contours of the 963 cm- ’ band are observed. The former complex is uniquely assigned to the gauche, the latter to the cis conformer, on grounds of both TMB and NIR conversion experiments. This raises the question whether the 953 and 943 cm- 1bands are site splittings of the same vibrational transition or are to be considered as two different transitions of gauche MVE. Analysis of the line parameter data strongly supports the first alternative: (i) plotting of ln(A(953 . . . 941)/A(963)) or ln(A(953 . . . 952)/A(963)) or ln(A(953 . . . 949)/ A(963)) vs T& yields approximately straight lines with the slope falling into the (statistical) error interval (RMS) of AH’Cr,)/R. (ii) the plot of In (A(945 . . . 941)/A(963)) vs T;i yields a slope outside the interval AH”(T,)/R f 2 RMS. (iii) the ratios A(949 . . . 941)/A(953 . . . 949) and A(949 . . 941)/A(953 . . 952) are essentially Tindependent (1.05(20) for TE (298, 922 K). Figure 8b shows NIR photoconversion of c to g to populate predominantly sites centered near 943 cm-‘, which now form a narrow band contour. Besides it leads to partial population of sites near 953 cm- 1 with a band contour similar to that formed by TMBs. It remains to rationalize formation of the broad distribution of sites between 950 and 942 cm- r by TMBs. More substantial arguments require consistent force field (CFF) model calculations of the system

334

TIMOTHY BEECH et al.

MVE: ArKy (K, b 364), providing qualitative knowledge of the dependence of the normal frequencies of MVE substituted in imperfect Ar crystal fragments. Regarding results obtained from CFF calculations for other A: M systems [28] one may suppose the mode localized at 953/943 cm-’ (gvl,ayL (CH:CHJ, cf. Section 4.3) to be particularly sensitive to defects of the matrix crystal produced by TMBs, owing to its dependence on torsional the angle u = + (C . O-C:C). The torsional potential associated with this angle is predicted to be very flat between the two g isometric structures (ue x 150,210”). Interaction between MVE and the argon atoms possibly leads to a wide distribution of MVE:Ar structures with u: ~(150-210~) and, as a consequence, wide distribution of normal frequencies of the v,, band and possibly other modes. The experimentally observed line contours with the system MVE:N, support such an interpretation. Under the deposition conditions of this work, the band contours of MVE isolated in nitrogen are much more complex and appear to be superpositions of many narrow lines (cf. Fig. 6 and Table 4). Since N2 appears to be a more rigid matrix crystal, interaction MVE-N2 may lead to wider variation of u: of MVE:N,. As a final point estimates of the barrier to rotamer conversion of MVE:M, M = Ar, Ng should be discussed. According to statements made above, the barrier of cis to gauche conversion cannot exceed x 31OOcm-‘, i.e. the quantum of NIR radiation inducing the process; on the other hand it must be larger than 1450 cm- ‘. Quantum chemical calculations on the MP2/6-31G level [24] predict a barrier of V(90) x 1710 cm-’ (denoting the torsional potential by V(u)) referred to cis with V(0) = 0. One may therefore conclude that 3000 cm-’ quanta readily (in principle) may induce c + g conversion by transfer of vibrational energy from singly excited CH-stretching modes to (C . O-C:C)-torsional states lying above the barrier. Experiments aiming at hole burning and/or g + c photoconversion by CO* laser irradiation of the band profile of the transition gv,,ay!l (CH,) at 1088.97 cm- ’ suggest the barrier to exceed the lowest gauche level by more than 1090 cm- 1, consistent with the finding that radiation with 3 c 14OOcm-’ does not induce any detectable photoconversion. According to quantum chemical prediction the barrier to g + c conversion amounts to V(90”) - V(1SO’) zz 780 cm-’ for the free molecule. Hence either the barrier to g + c conversion of MVE:Ar is > 1090cm-’ or the transition probability for energy transfer from singly excited gvl,ay/l(CHP) to torsional states above the barrier is too small. The first alternative appears to be thecorrect one, since in analogous experiments with CH2FCH2F the barrier to t + g (unstable + stable) conversion has been found to be higher by x 400 cm- ’ for the matrix isolated molecule than for the free molecule [15]. For MVE:Ar the increase of the barrier by the matrix is expected to have a similar value.

Qualitatively similar conclusions are arrived at if in place of the quantum chemical potential function the torsional potential published by DURIG and COMFTON is used. The latter proposed barriers to c + g and g + c conversion of 22 15 and 1774 cm- ’ (for free CDsOCH:CH2), respectively. In this case only photoconversion c + g by 3100 cm- ’ photons is expected. Excitation of N(C: C) (cv,a’N(C: C) and gv,aN(C:C)) near 1630cm-’ should be ineffective, whereas excitation of this mode might be effective, if the quantum chemical potential function were more correct. The presently available data do not allow one to single out relative quantum efficiencies of CH,stretching modes on one hand and v(:CH) modes on the other hand. For both types of v(CH) modes transfer of vibrational energy to torsional states is quantum mechanically allowed. The same holds for the N(C:C) stretching mode. The quanta of the latter would be sufficient to pass the free molecule over the g + c barrier (quantum chemical barrier 780 cm- ‘). Study of this mode therefore opens the possibility of obtaining new experimental information about the potential function. The system MVE:N, behaves similarly to MVE:Ar with regard to photoconversion; however the conver-sion rate amounts to only about 35 % of that observed with the latter system. Figure 6 furthermore shows the integrated absorptions of vibrational transitions of the conformers in both matrices to have a similar magnitude. Owing to the considerably stronger scattering of N, matrices, direct comparison of conversion rates in the two matrices requires appropriate correction for matrix scattering, which however turns out to be rather complicated and critical even within the Kubelka-Munk model. The band contours observed at the longest irradiation times shown in Figs 5 and 7 correspond approximately to the photostationary state reached under the conditions available in this work. The question of one-photon vs two-photon photoconversion processes may also be approached by means of the ratio of initial slopes of growth of the integrated absorption of any particular absorption band induced by 460mW and 230mW radiation power dA(t; 3,,460 mW) dA(t;i+, 230 mW) r= dt dt 1=0 II=0 I Averaging over all bands studied kinetically by NIR irradiation experiments yields r Byx l-8(4); this again shows the photoconversion to be a onephoton process. Though analysis of the growth curves provides some information on band contours and quantum yield of particular bands, no further discussion will be presented here.

4.3. Assignment of matrix spectra of cis and gauche MVE:Ar(N,) Considering

all internal

modes of MVE as in-

Infrared

spectroscopy of MVE

335

analysed mainly FIR bands for derivation of structure (of gauche) and potential to rotation about the C.O-C:C bond. Combination of the TMB and NIR photoconversion experiments leads to the assignment of nearly all vibrational modes of both s-cis and s-gauche MVE:Ar as proposed in Table 8. For MVE: N2 one arrives at the same assignment with frequency shifts of l-3 cm-‘; therefore no explicit vibrational data for the latter system will be given. The assignments rests on general empirical rules for group mode frequencies [30] and furthermore parallels the assignment of cis MVE given by IGNATIEV et al. [29]. As a rule analogous mid and far i.r. fundamentals of conformers differ in frequency by 5-30 cn- ‘, though intensity (1+/aQ 1’) and group mode composition (e.g. PED) may alter by factors up to 10. In Table 8 only approximate maxima of band contours are listed; components of band contours of vibrational transitions in the frequency range 1400-800 cm- ’ are collected in Tables 2, 5, 6; for fundamentals outside this range either such information is not available (> 14OOcm-‘) or will not be reported ( c 800 cm- I). The following comments are restricted to particular

finitesimal one would expect 24 fundamentals, two of which (T(CH,) and T(C.O-C:C) are torsion type vibrations. The fundamentals of s-cis may be classified in terms of the covering group C, as 16 a’ + 8 a” species. For an s-gauche conformer all infinitesimal modes classify as a species, whereas for an s-trans conformer symmetry classification is the same as for the s-cis conformer. In the assignment which follows, both classifications will be given, cf. Table 8, but discussion is kept in terms of the s-gauche conformer. The experimental data presented above allow a fairly complete assignment of the vibrational modes of both cis and gauche MVE. Vibrational assignments based on gas phase i.r. spectra were reported by OWEN and SHEPPARD[22] and more recently by IGNATIEVet al. [29]; the latter work included liquid phase Raman work on vibrational spectra and normal coordinate analysis of MVE and 2 deutero isotopomers. Far i.r. work reported by DURIG and COWTON [23] has been mentioned already. The earlier body of work has yielded only fragmentary data on the vibrational spectrum of possible less stable conformers. IG~ATIEV et al., for instance, assigned three mid i.r. bands of a second conformer, whereas DURIG and COMPTON

Table 8. MVE:Ar assignment of i.r. spectra of s-cis and s-gauche conformer Frequency* (.&- 1 3132 3080 302On 3014sh 2960 2945 2908 2860 2847 2830 21405 2106 1873 1870sh 173s 1675 1665 1657sh 1644 1641 1625 1618sh 1614 1610 1595 1523 1514 1469 1466 1458 1455sh 1446 1398~~ 1389 1321 1311vw

TMBt

NIRt

ndt

ndt

s-gauche

s-cis cv,a’v(: CH) cv&, (:CH,)

gv,av(:CH) gv,av,(:CH,)

(v,a’) (v*a’)

cvJa’v, (:CH,)

gvw,(:CH,)

(w’)

cvl +“(e)v, (CH,) cw’(e)v,. (CHJ 2v,,A’, v, + VI& 2v,A’

gv.&+,(CH,) gv&)v, (CH,)

(vl &) (w’)

cvSa’v,(CH,)

gwv (CH,)

(v&

FR v,aN(C:C)

(w’)

FR?

t 1

t 1

t

t

H,O:Arll H,O:Ar H,O:Ar

7

i 1

i

t

t

:

I

nd nd

nd nd

t

t

FR v,a’N(C:C) 2v,,0w(:CH,)

H,O:Ar

kd’(eP,(CH,)

w@)G,(CH,) wW,(CW

w’W,(CW vsa’G,(.:CH,) v&UCHJ v,,a’S(C:CH)

v,,&(:CH,) vl &,(CHJ v,,ab(C:CH)

bv’)

336

TIMOTHY BEECHet al.

Table 8. (Contd) Frequency* cm-’

TMBt

NIRt

1238

t

t

1216

1

1

s-cis

s-gauche

w%(c/“\c)

1200 1188 1156~~ 1136 1089

wvb WV

vlsay!l V-U

+ NCW + A (COC)

1008 963 953 943

v13a’~,CCH2) v,,a”yL(CH:CH,)

bw”)

892

1

873

T

827 817 812 701 692 587 521 330vb 310vb 272 25Ovb 220 215

65 ,a’)

1

wfy,(:CW V,#"yi( :CHJ

v,,w,CCW

ha”)

v,,aA(COC)+A(OC:C) v,,aA(COC) + A(OC:C)

(vl 4) (V,&‘)

v,,aT(C.O-C:C)

(h3a”)

v,,a’A(COC)+A(OC:C)

v,,a’A(COC) +A(OC:C) ? v,,aT(C.SC:C) * ? v,,a”T(CH,)

*Error of frequency for band contours with pronounced peak: f 0.5 cm-‘.

t t( 1) band intensity increasing (decreasing) with increasing Knudsen cell temperature and increasing fluence of NIR irradiation. IGrowth behavior in TMB and NIR conversion experiments not detectable. QBands not assignable on ground of available data. llyv,y,, y, denote wagging, twisting and rocking modes, respectively, of CH2 and CH5 groups, yY,yldenote in-plane and outof-plane modes respectively, N, A denote heavy atom stretching and bending modes respectively. ~/VW,sh denote very weak band and shoulders of bands, vb: broad band in FIR region ( 3 10 cm-’ HWHM). **Second mode notation in parentheses applies if second conformer has s-tram conformation.

aspects of the assignments and may serve to complement Table 8. (i) In the v(CH) region where for each conformer three modes with P 2 3000 cm- ’ (v(:CH)) associated with the vinyl group and three fundamentals with 9 < 3000 cm-’ (v(CHJ) modes) are to be expected, only nine bands were identifiable with the spectral resolution available. The region below 2980 cm-’ probably is complicated by Fermi resonances (FR) of bending and stretching modes of the methyl group. Since no reliable discrimination appears possible at the present time between v(CH) stretches of different sconformers, the v(CH) fundamentals of cis- and gauche-MVE were assumed to coincide. Empirically the symmetrical stretching mode v, (:CH,) is generally found to be weak and to lie slightly below 3000 cm- I, however no absorption was detected near this

frequency. Tentatively this mode is assigned to the relatively strong band near 3020 cm- I. (ii) In the 1700-1600 cm-’ region the spectra show a complicated structure most probably due to Fermi resonance of the C:C stretching fundamental with 1st overtones of the out-of-plane modes (rl(:CH)) of the vinyl group, site splittings of both fundamental and overtones and overlapping with spurious bands of Hz0:Ar[31]. Since the latter exhibit bands with temporarily variable intensity, sorting out of the Hz0 bands was not always unique: as a consequence coinciding H20:Ar bands are included in Table 8. Though the assignment given seems to be unambiguous concerning attribution of observed bands to either conformer, complete interpretation of the 1700-1600 cm- 1 bands appears not possible without use of spectra of “C- and ‘*O-isotopic modifications;

Infrared spectroscopyof MVE

obviously interpretation involves not only well known binary resonances mentioned already, but also further combination tones such as ternaries. (iii) 1500-13OOcm-’ range: empirically one would locate in this range the bending modes of both vinyl (in plane) and methyl groups, based on work published by THOMPSON et al.[32, 33, 34, 353. Accordingly one arrives at the assignment of the typical light atom bending modes suggested in Table 8. The most problematic aspect concerns the location of the &(:CH,) bending modes of both conformers, which conventionally should be placed in the range 1420-1400 cm- ‘. However no bands were found in this region, and instead two bands near 1398 and 1389cm-’ were detected. Tentatively these are identified with the 6, (:CH,) and 6,(CHJ) modes, assuming the two conformers to have coinciding frequencies. Identification of u,,a’S(C:CH) of the g conformer near 1311 cm-’ is based on the evidence documented by Fig. 6 and Table 4. The evidence shows the counterpart of this mode of the cis conformer to exhibit a fairly complex contour near 1321 cm- ‘; this probably applies also to gvi2u, but the band proved too weak for measurement of its contour (possible overlap with cv,,a’G(C:CH)). (iv) 130&200 cm- 1 region: following the rules adopted, assignment in the fingerprint region appears straightforward. The vlr,a”y,(CHo) mode of the cis conformer (1156 cm-‘) is quite weak, so neither detailed band contour measurements nor quantitative determination of the temporal behavior in NIR photoconversion experiments were feasible. Qualitatively its attribution to the cis conformer is unambiguous. On the other hand neither one of the two fundamentals cv12u’ nor gv,,u(gv,,u’) could so far be located; these are the only fingerprint modes of the conformers, for which this work provides for no attribution. Either they are too weak to be detected in diluted matrices and/or they are overlapped by strong and complex band contours. For instance the cv,,u’$ (: CH2) mode near 1008cm- ’ shows both complex band contour, irregular temporal behavior in photoconversion experiments and yields unusually high values for AH” for all reasonable pairings with other bands. However the accuracy of intensity measurements achieved in the present work allows no definite conclusions. Another unusual aspect of both MVE: Ar and MVE: N, spectra concerns identification of the mode gv,,ay,_(CH:CH2); this has been discussed already in connection with photoconversion and site structure (Section 4.2). Assignment of the fundamentals below 400 cm-’ is less well established, mostly owing to broad and little structured band contours. Modes of matrix isolated molecules (below 3OOcm-‘) often exhibit this behavior [21]; one possible interpretation of the phenomenon in terms of consistent force field modeling has been put forward by GUNDE et al. [28]. A further complication stems from the fact that the gauche conformer is expected to have a medium barrier

337

to internal rotation of the methyl group. Though intermolecular interaction of methyl groups with the matrix species might increase this barrier considerably, no evidence for its presence near or above 200 cm- ’ has been found.

5. CONCLUDING REMARKS 5.1. Assignments for second conformer

IGNATIEVet al. [29] proposed assignments for only three fundamentals, located at 1139, 835 and 525 cm- ‘, of the less stable conformer. This is confirmed by this work, but there are some revisions to be made in the vibrational assignment of the cis conformer. First the band located near 1090cm-i (1089cm-’ for MVE:Ar) definitely belongs to the gauche conformer. Second, assignments of the S(CH3) modes differ considerably with regard to frequency; regarding the complexity of the gas phase spectra and the rather low intensity of the matrix spectra in the 1450 cm-’ range no definite proposition to alter the gas phase assignment will be made. Finally gas band assignment in the N(C:C) mode region 1600-17OOcm-’ is quite incomplete and lacks all details of the Fermi resonance dominating this part of the i.r. spectrum. In spite of rather complete vibrational data of the g conformer the question of its r, structure (gauche or truns) is still open, though a number of details observed in its i.r. matrix spectra suggest to assign it as gauche. Similarly there are still lacking reliable valence force fields for the two conformers, which would allow reliable statistical computation of the cis-gauche equilibrium. This together with the thermochemical quantity AHo and integrated absorption coefficients for some pairs of modes should allow one to settle the structural problem of the less stable conformer. The latter combined with integrated absorption coefficients also should make it possible to rationalize the widely differing intensities of analogous fundamentals of the two conformers.

5.2. Band contours and structure of M VE: M Detailed data on components of band contours of MVE isolated in solid Ar or N, rise a number of questions concerning the physical state of such systems, intermolecular interaction potential, etc. Most of these problems are still open, but may be of some interest for solid state oriented matrix spectroscopy, vibrational energy transfer to the doped matrix crystal, the defect structure of the latter and others. 5.3. NIR photoconversion As already pointed out in Sections 4.2, 4.3 the high sensitivity of photoconversion suggest the MVE:M system for further study, in particular for detailed measurement of quantum yields of excitation of the six CH stretching modes by tuned laser radiation, or of the quantum yield for conversion by one- and two-photon

338

TIMOTHY BEECHer al

excitation of the C:C stretching mode. Furthermore the theory of the conversion process in itself offers some open problems in molecular quantum mechanics.

support. Furthermore we wish to thank K.

REFERENCES

5.4. Line shape analysis Regarding the extensive application of the LSA procedure in this work the following comments appear in order. (i) By experience the resonance frequency 3, of significant components is determined within 0.1 yLsA i.e. within 10 % of the apparent line width; broad bands (yLSA> 1 cm-‘) are less well localized. (ii) Integrated absorption A,,, of a component is usually determined with < 10% error; for strongly overlapped and for weak components the error may be larger. Errors of A,,, depend on the choice of the base line; as a rule linear and quadratic base lines lead to differences of a few per cent. As reported in recent work [ 191 A,,, is corrected by the deconvolution process often by less than 10-15x; regarding the errors inherent in intensity measurements and in the LSA process correction of A,, is sometimes not warranted. (iii) Line shape parameters yLSAof particular components are determined within < 10% by the LSA process; again the error might be larger for broad bands. Uncertainty of choice of the base line mostly affects yLsAby less than 2%. (iv) In this work only LSA results obtained with Lorentzian shape function have been reported. As pointed out earlier, this shape generally reproduces the central part of isolated bands well (< 0.01 discrepancy of optical density) but leads to too high optical density in the band wings. The latter however depend critically on the choice of the base line; furthermore apparent line contours of Lorentzian bands produced by finite slit width measurement have higher optical density in the wings[l9]. In the examples reported above the RMS deviation of the LSA fit from observed optical density amounted to less than 0.01. Gaussian shape functions generally were found to lead to (statistically) significantly better fits than Lorentzian shapes, in particular the wings are better reproduced. However Gaussian LSA of complex contours notoriously yields one dominant broad band providing fdr the fit in the wings. This phenomenon has been considered as an artefact and suggested systematic use of Lorentzian instead of Gaussian shapes. (v) The present work may serve as an example for application of LSA to description of complex band contours (in analogy to gas phase band contours), for disentangling overlapping bands, site splittings, and for discrimination of vibrational transitions of different conformers. Acknowledgement-The authors wish to thank the Swiss National Foundation (Project No. 2.219-079, 2.612-0.80, 2.079-0.81) and Messrs. Sandoz AG, Basel, for financial

G~~NTHARD for

typing,the manuscript.

[ 1] Hs. H. GUNTHARD, J. molec. Struct. 80, 87 (1982) and references cited therein. [2] M. SQUILLACOTE, R. S. SHERIDAN, 0. L. CHAPMAN and F. A. L. ANET, J. Am. them. Sot. 97, 3244 (1975). [3] R. GUNDE and Hs. H. GUNTHARD, Spectrochim. Acta 39A, 315 (1983). [4] A. SERRALACH, R. MEYERand Hs. H. GUNTHARD, J. molec. Spectrosc. 52, 94 (1974);A. SERRALACH and R. MEYER,J. molec. Spectrosc. 60, 246 (1976). [S] A. J. BARNESand G. C. WHITTLE,Proc. 12th Eur. Conar. Melee. Spectroscopy, Amsterdam 1976, p. 373, where a review of earlier %rk is presented. f61 R. T. HALLand G. C. PIMENTEL. J. them. Phvs. _ 36.1889 L A (1973). [7] J. S. SHIRKand P. A. MCDONALD,J. them. Phys. 77, 2355 (1982). r8l __ H. FREI; TAEKU HA, R. MEYER and Hs. H. G~~NTHARD, Chem. Phys. 25, 271 (1977). r91 M. PERTILLA. J. MURTO.A. KIVINENand K. TURUNEN. L d Spectrochim.Acta 34A, b (1976). [lo] M. PERTILL&J. MURTO,L. HALONEN,Spectrochim. Acta 34A, 469 (1978). [l l] L. HOMANEN,Spectrochim.Acta 39A, 77 (1983). [12] J. POURCIN,G. DAVIDOVICS, H. BODOT,L. ABOUAFMARGUIN and B. GAIJTHIER-ROY, Chem.Phys. Lets. 74, 147 (1980). [13] J. S. SHIRK,Symposium on Molecular Spectroscopy, Culumbus, Ohio, June 1983;W. F. HOFFMANN, III and J. S. SHIRK,J. them. Phys. 78, 331 (1983). 1141H. TACHE&HIand M. TASUMI,Chem. khys. 70, 275 (1982). Chem. Phys. 85, 1 Cl51 P. FELDERand Hs. H. GONTHARD, (1984). Chem. Phys. Len. 88, Cl61 P. FELDERand Hs. G~~NTHARD, 473 (1982). and Hs. H. GUMHARD,J. Phys. 1171P. GRONER,I. STOLKIN E. Sci. Instr. 3, 261 (1970). R. GUNDE,Thesis No. 7098, ETH Ziirich (1982). [E] R. GUNDE, 1. KELLERand Hs. H. GUNTHARD, Spectrochim. Acta, in press. PO1 G. KORT~~M,Reflectance Spectroscopy, Principles, Methods, Applications. Springer, New York (1969). and Hs. H. G~~NTHARD, C’hem. c211 P. HUBER-WALCHLI Phys. Lett. 30, 347 (1975); M. SQUILLACOTE, R. S. SHERIDAN, 0. L. CHAPMAN and F. A. L. ANET,J. Am. them. Sot. 97, 3244 (1975); thermal molecular beam trapping of unstable conformers has systematically been used for determination of conformer equilibria cf. e.g. P. HUBER-WALCHLI and Hs. H. GUMHARD, Spectrochim. Acta 37A, 285 (1981); P. FELDER,T.-K. HA, A. M. DWIVEDI~~~Hs. H. GUNTHARD,C~~~. Phys. Lat. 73,483 (1980); R. GUNDEand Hs. H. GUNTHARD, Spectrochim. Acta 39A, 315 (1983). Trans. Faraday Sot. 60, PI N. L. OWENand N. SHEPPARD, 634 (1964).

1. them. Phys. 69, c231J. R. DURIGand D. A. C. COMPTON,

2029 (1978);these authors presented evidence based on FIR spectra for the unstable conformer to possess gauche r, structure. J. molec. c241 R. H. NOBEL,L. RADOMand N. L. ALLINGER, Strut. Theo. Chem. 85, 185 (1981); the reader should consult this work for references relating to earlier quantum chemical and molecular mechan& work. and Hs. H. GUNTHARD, v51 H. FREI,R. MEYER,A. BAUDER Molec. Phys. 32, 493 (1976).

Infrared spectroscopy of MVE [26] H. FREI, P. GRONER, A. BAUDER and Hs. H. G~INTHARD,Mokc.Phys. 36,1469 (1978). [27] H. FREI, A. BAUDERand Hs. H. G~IINTHARD, Molec. Phys. 43, 785 (1981). r281 _ * R. GUNDE. P. FELDERand Hs. H. G~~NTHARD.Chem. Phys. 64, 313 (1982). f291 I. S. IGNATIEV.A. N. LAZAREV.M. B. SYIRNOV. M. L. ALPERTand B. A. TROFIMOV,‘~. molec. Struct.‘72, 25 (1981). [30] L. J. BELLAMY, The Infrared Spectra of Complex Molecules. Chapman & Hall, London (1975); the reader might consult this book for quotation of numerL

.I

[31] [32] [33] [34] [35]

339

ous references to work contributed to this field by H. W. THOMPSONet al. R. L. REDINGTONand D. E. MILLIGAN,J. them. Phys. 37, 2162 (1962). H. W. THOMPKJNand P. TORKINGTON,Proc. R. Sot. A184, 3 (1945). R. E. RICHARDS~~~ H. W. THOMPSON,Proc. R. Sot. A195, 1 (1948). H. W. THOMPSONand P. TORKINGTON, Trans. Faraday Sot. 42,432 (1946). H. W. THowsoNand D. H. WIFFEN,J. &em. Sot. 1412, (1948).