Singlet → triplet transitions in methyl-substituted ethylenes

VoIiume 36, number

Received

1

CHEMICAL

-

PHYSICS

15 October

LETTERS

1975

1 July 1575

The low ener,oy elecrron impact energ-loss spectra ofetk.lene and its six rne~~la~ed derivatives h;lve been studied at impact energies from 15 eV to 80 eV, and at sc&tetig angles from 10” to SO”. In each molecule, the N -+ T singlet -+ triplet transition has been detected with a nlaximum intensity at an ener_g loss which shifts from 4.32 eV in ethylene to 4.10 eV in tetr~me~lylethylene. No fe:ttures which WR be assigned to the tt: - TR singlet -+ triplet Rydberg transition or the N --+R* (TI-+ LS) singlet -singlet Rydjerg transition were detected.

spectra of.ethylene and its methyl-substituted derivatives has permitted a compsiison of the iocations and band system profiles of the 14+ T [I] sir&et -+ triplet transitions. Various members of this series of molecuLes have been studied .previouilji be electron impact [2-91 , ion impact [ 10, 111, and optical spectroscopy [12-151, but the present work is the first systematic investigation of the singlet + triplet absorption of all members of this series.

and trimefhylethylene is available from oxygen-perturbed or CH2 &z-perturbed optical absorption spectra 1141 of these molecules as well as from the normal optical absorption spectrum [ 151 of liquid isobutene. These absorption spectra show, at most, a shoulder on a broadly rising background. For the perturbed spectra [ 141, the N -+ T band system profiles were obtained by a subtraction procedure, and must be viewed with caution. The present study was undertaken to obtain, with one experimental technique, high-quality band-

Ethylene is the simplest olefm, agd a thorou~ understanding of its e!ectronic spectrum and those of its

shapes, as well as the locations of the Rla~~lurn intensity absorptions of these N + T transitions. The band-

methylated

shapes are important

2%~ experimental

electron

impact

investigation

of

,’ the e’iectronic excitatiofi

derivatives is a prerequisite

for the under-

stafiding of the electronic spectrs of more complicated unsaturated systems. Satisfacrory locations and bandshapes for the N --z i T transition have been determined for ethylene [3,59,111 1propene [2,9] , cis 2-butene (7,9,11] , tram 2butene [7,9,1 I], and tetramethylethylene [7] by var-

iable angIe electron impact [Sj , threshold electron imPact _

[3,6,7,9]

, and ion impact

techniques

[l 1] . Some

crude information about this transition in isobutene 7 Work supportkd in part by the United States Ener,gy Research and Development Adm~is~ration Report Code %ALT-767P4-137. G’. Work performed in partial hlfimcnt of the requirements for, fh Ph;D_‘?ieg~ee in Chemistq- at the Crtlifornia Institute ‘. :. . af Technbld~)r.,

? ~o~~ibu~o~ . .

,_56.

:._.

’

NoAZ53.

‘,,.

‘,_

..

1

the analysis of photoc~lemic~

most easily compared with theoretical calculations for these systems. An additional goal of the present investigation

was to search

For the N + TX singlet

-+ triplet

Rydberg transition [1,9,15,19,20] possibly anaiogous to the feature recently detected at 6.4 eV in vinyI fluoride

[21]

(monofluoroathylenF3).

It is well documented

the method of.Low ener,T, variable-angle electron impact spectroscopy [8,1S,21-231 is well suited foi the detection of spin-forbidden transitions in both atoms and complex mofecules. In this paper, we. repoit results obtained from the low ener,v-loss region of ethylene, propene, isobutene (2methylpropene), cis and PQIZSSbutene, trimethylethylene (2-methyl-2-butene~, and tet~met~~lethy~en~, that

__

.,

..-.._ ‘..

for

electronic ener,q transfer experiments [i&-18], bvhiie the maximum intensity locations are the q~~antities

‘.

Yolune

36, number

CHEXlICAL PHYSICS SETTERS

1

15 October 1975

(2,3-dimethyl-2-butene) obtained at impact energies, from 15 eV to 80 eV. The scattering angIe. 0, varied from 10” to 80”) and the apparatus employed has been described previously [8,X,23]. In brief, elec-

by the low-energy tail of the N-t V band system in ethylene, propene, isobutene, and tram 2-butene (fig. 1). With increasing methyl substitution, however, the N -+ R trar.sition shifts to lower energies more rapid!y

trons

than dces the N + V one, and the two features are readily distinguishable in trimethylethylene and tctramethylethylene (fig. 1). The N + R transition, which has

E,,

are emitted

from

a t’ungsten

filament,

and pass

through a multistage gun, a hemispherical electrostatic energy monochromator, a flexible bellows scattering chamber, an ener,? analyzer identica1 to the monochromator, and are detected with a continuous dynode electron multiplier. The output pulses are stored in a multichannel scaler. The energy-loss spectrum, at a fixed impact energy and scattering angle, is swept repeatedly for an average data accumulation time of 46 hours. Sample pressures in the scattering chamber are typically 3-6 mtorr? and incident electron beam currents into the scattering chamber average 50 EA. The energy resolution, as measured by the full width at half maximum of the elastically scattered peak, was adjusted in this study to lie between 0.10 eV and 0.15

been referred to in the literature as the “mystery band” [25] , has now been assigned [ 1,15,20,26] with reasonable assilrance to a pi + 3s Rydberg excitation. At moderate impact energies (40-50 eV), the ratio of the area under the N + T transition to that under the optically allowed N + V transition increases by a factor of approximately 40 as the scattering angle increases from 10’ to 80”. At lower impact energies (15 -25 ev), this ratio increases by approGmately one order of magnitude as 19increases over the same angular range. In addition, for most scattering angles, this ratio

is larger at low impact energies than at high ones, This

eV.

angle and impact

The ethylene and mono- and di-substituted compounds were obtained from Matheson Gas Products.

?I -231

The trimethylcthylene

The position of the maximum intensity of the N + T transition in each molecule studied is listed in table 1. The locations of these maxima are estimated to be accurate to within 0.05 eV. The observed FranckCondon limits of the N--f T transition ara also given

and tetramethylethylene

used

were obtained from the A!drich Chemical Company.

The stated purity of the ethylene sample was 99.5%, while that of the other substances was 99.0%. AU samples were subjected to a liquid nitrogen freeze-pumpthaw cycle prior to use, in order to remove volatile impurities. Fig. 1 displays the low energy Ioss portions of the energy-loss spectra of these molecules obtained under a variety of impact energies and scattering angles. The band system profiles do not change significantly over the range of impact energies and scattering angles studied, and therefore the profiles in fig. 1 can be compared with each other. The electronic spectra of all methylated ethy!enes below S eV energy loss are rou&Iy

similar in their general appearance. The Iowest inelastic feature in all members of the series is a weak transition with maximum intensity between 4.1 eV and 4.3 eV, analogous to the N + T transition [ 1J in ethy1en.e. At higher energy loss there is a strong feature, with maximum intensity between 6.6 eV and 7.6 eV, which is analogous to the singlet + singlet N + Y transition in ethyl. ene, and which is commonly assigned [ 1,241 to a TT _?T* excitation. Between the N + T and N + V transitions is a moderately strong feature,designated h’ + R [I], which is superimposed on and partially obscured

energy

the spin-forbidden

dependence

character

confirms

[S,lS,

of the N--t T

transition.

in table 1. These limits represent the detected onset and end of the observed absorption, and are repro-

ducible to within 0.1 eV. It should be noted that an absorption onset in electron impact spectroscopy corresponds

to the O-O band

of the electronic

transition

only if the Franck-Condon factor for this band is larger than a certain minimum value determined by signal/noise considerations. The N + T transition peak locations (table 1) show an overall downward shift of 0.22 e’J in going from ethylene to tetramethylethylene, in qualitative agreement with semiempirical calculations [ 19,271 . The transition energy is, within experimental error, a decreasing linear function of the number IZof methyl substituents, as shown in fig. 2. A linear, least-squares fit of this function furnishes the expression (4.33 5 O.Ol)-(0.056 r O.O04)!r for the trvlsition energy in eV, with standard deviations indicated. For comparison, we also list in table 1 the N + T maximum intensity tra.nsition energies obtained by other investigators, as well as the theoretical results. 57

\iolume 36,number 1

CHEMICALPHYSICS LETTERS

15 October 1975

Ethylene

Cis 2-Butene

Propene E,= 60 eV 8 = 60”

E,=40eV

e= 700

Trimethylsfhylene

Trons

2- ktene

Tetramelhylethylene Eo= 40eV

E,=20eV e =a400

0

AE CeV) Fig. 1. El&on

energy-loss

spectr;; of ethylene and its six methyl-mbstituted

q

60”

1 I

BE (eV) derivatives in the 3-8

eV energy,loss

region.

Volume 36, number 1

1.5 October f97f

CHEhfICALPHYSICS LEl-iERS

n Fig. 2. Dependence of the N-t T’ vertical transition energy

A!?on the number n of methyl substituents. The awe fepresews a Ieut-squares str2ight line fit to the experimenhl data. There is a considerable spread (OS eV) among the N + T peak values listed in tab!e 1 for ethylene, with most of the. earlier 12,131 or lower resolution [2,4,lCl] experiments yielding values of 4.6-4.7 eV, while more recent and higher resolution techniques [S-9,1 1) give values between 4.2 eV and 4.4 eV, the present one being (4.32 rt 0.0.5)eV. Although the 4.6 eV value obtained by Evans [13] is the most widely quoted one, it is often overIooked that the highest band in the N + T system actually detected by him was at 4.54 eV, and that an accurate band system profIile could not be obtained because sti?l higher bands were hidden izder intense contact charge-transfer absorption. It is quite possible that if this absorption could be subtracted from Evans’ spectrum, the most intense band would be either the ose he observed at 4.42 eV & the one at 4.30 eV, i7 closer agreement with our results, which give a more accunta intensity profde of the band system. The size of the bathochrom~c shift in the N + T transition energy from ethylene to tet~rnethy~et~~lene is small compared to the corresponding 1.O eV and I.6 eV shifts [ZS] in the maximtim intensity energies of the N -* V and N + R trznsitionq, respectively. This nlative ~sens~t~ity of the N 4 T transition energy to methyl subsptution in the (TeIec$mn framework of ethylene agrees with the results of our.p~e~o~s,s~d~ [21] of the analogous transition,iri the fiuoroethylenes. ,meie results add support to the C--ST separation approx., :

19

‘. Vclume 36, number 1

CHEMICAL PHYSICS LETTERS

irnation commonly used in theoretic;al calculations of this excitation energy; and imply that the charge disiribution in the N and T states is similar and is sin!ilatly ‘affected by methyl ~ubsti~ut.io~ ti the CJcore. Furthermore, the goad agreement between the experirnentai and theoretical exditation energies (table 1) confirms the model [24,26] of,the N -+ T transition as 2 valence7r--‘ii* exciiation. .. In none of our spectra of these seven’molecules

.type

do we find evidence of a feattirlt.anaIogous to the weak singlet .+ trip!et transition we have recently reported at 6.4 eV in vinyl fluoride 117-11,and which Dance and MWker f9 ] suggest may occur in their trapped electron spectra at 6.5 eV in ethylene, and in the 5.8-6.4 eV region in propene. In addition, we do not observe any bf the +arious~weok transitions which have been reported

[l :I 5,291 for the liquid meihylated ethylenes to lie in the 4.5-5.5 eV excitation energy range and which have been tentatively assigned to N -+ TR singJet + triplet tra+tions [i ,15,19,20] and N -+ R”(s; + o”) singlet +singlet _Rydbergtransitions [15:20] _ Such transitions,

if not

due to impurities, are either considerably

weaker

than the N + T transition under the present experimental conditions, or are so close (< 0.5 eY) to the N -+ R Rydberg singlet + singlet excitation as to be obscured by it. In summery; we ilaGe obl ained accurate locations and Fran&-Condon band limits for the N + T singlet .-+ triplet transitions in ethylene and its six methylated derivatives..The N + T transition energy is,relatively insensitire to the degree of methyl substitution, although a slight bathochromic shift is observed with increasing substitution. The experimerkil energies agree weli with recent theoretical calculations and are consistent with

the validity of theoretics models vjhich predict a simih- charge distribution in the N and T states. Finally, we find no evidence of a transition to a TR triplet Rydberg state.in‘the 4.5-6.5 eV energy loss region, or a weak R* singlet Rydbets state in the 4.5-5.5 eV en&xEy loss,region

in any of these molecules.

Rkferencs

fl].

A.J..Merer

-. atid R.Sl hfu%&cn, Chem. F&v. 69 (1959) 639. ..

‘;;

.’ :.

_.

;,y

-‘.

I5 October 1975

[ 2j !I. Kuppermann and L.&l. Ruff, Ciscussions Faraday Sod. 35 (1963) 30. [3] C.R. Bowman and W.D. Miller, J. Chem. Phys. 42 (1965) 681. f4] J-P. Doering, J. Chem. Phys. 36 (1967) 1194. [.5] J.P. Doerins and A.J. WiUiams III, 3. Chem. Phys. .a7 (1967) 4180.

f6] H.H. Brongersma, J.A. va der Hart and L.J. Oosterhoff, in: Fast reactions and prirnnry prctcesses in chemical kinetics, ed. S. Claesson (interscience, New York, 1967) p. 211.

[7] 1I.H. Erongersma, Ph.D. Thesis. University of Leyden, &den, The Netherlands (1968). [ 81 S. Trajmar, J.K. Rice and (\. ~upper~lann, r\dvan. Chem. Phys. is (1970) 15. [9] D.F. Dance and I.C. Walker, Proc. Roy. Sot, A334 (1973) x9. [IO] J.H. hfoore Jr. and J.P. Doerins, 3. Cbrm. Phys. 57 11970) 1692. [ll] J.H. bloore Jr., J. Phys. Chem. 76 (1972) 1130.

[12] C. Reid, J. Chem. Phys. 18 (1950) 1299. [ 13J.D.F. Evans, J. Chem. Sot. (1960) 1735. 1141 M. Itoh and R-4. ~~uff~~en,J. Pbys. CItem. 73 (1969) 4332. [15] F.H. Watson Jr. and S.r. XlcClynn, Theoret. Chini. Acta 21(1971) 309. [ 161 hi.W_ Schmidt and E.K.C. Lee, J. Am. Chem. Sot. 9@ (1968) 5919. iI71 M.W. Schmidt and E.K.C. Lee, 1. .4m. Chem. Sot. 93 (1970) 3579. El81 0-A. hlosher, W.hl. Flicker and A. Kuppermann, J. Chem. Phys. 59 (1973) 6502. 1191 R.S. hlolliken, J. Chem. Phys. 33 (1960) 1596. WI F.H. W’atson Jr., A.T. Armstrong and S.P. hlcGlynn, Theoret. Chim. Acta 16 (1970) 75. [211 M.J. Copgiola, O.A. hlosher, WM. Flicker and A. Kupprrmannz Chem. Pilys. Letters 27 (1974) 14. [22J A. Kuppermnn, J.K. Rice and S. Trajmar, 3. Whys. Chem.

72 (19.68) 3894. [3_31 O.A. hlxhher, W.&LFlicker and A. Kuppermann, J. Chem. Pltys. 62 (1974) 2600. WI T-H. Dunning Jr., W.J. Hunt ancf W-4. Goddard III. Chem. Phys. Letters 4 (1969) 117. WI hf.B. Robin, R.R. Hart and N..4. Kuebler, J. Chem. Phys. 54 (1966) 1803. and W.E. Karnm~r, 1. f36j R.J. Buenker, SD. Peyetimhaff Cnem. Ptlys.‘S5 (1971) 514. 1271 N.L. Allinger, J.C. Tai and T.W. Stuart, Theoret. Chim. Acta 8 (1967) 101. unpub[aal \S.hl. nicker, a.A. Masher and A. Kuppermann, lished data. 1291 W.J. Potts Jr.; j. Chem. Phys. 23 j1955) 65.