Photoelectron spectroscopy of some thiocyanates, isocyanates and isothiocyanates

Journal of Electron Spectroscopy and ReZated Phenomena, 18 (1980) 179-188 0 Elsevler Sclentlflc Pubhshmg Company, Amsterdam - Prmted m The Netherlands

PHOTOELECTRON SPECTROSCOPY OF SOME THIOCYANATES, ISOCYANATES AND ISOTHIOCYANATES

B J M NEIJZEN and C A DE LANGE Chemzcal Laboratory, Free Unzoerszty, 1083 de Boelelaan, Amsterdam (The Netherlands) (Received 2 May 1979)

ABSTRACT Photoelectron spectra of some thlocyanates (RSCN, R = CHJ , Cz H5, n-C4 Hg ), lsocyanates (RNCO, R = C2 H5, n-C4 Hg ) and isothlocyanates (RNCS, R = CZ H5, n-C4 Hg ) have been measured, to study mteractlons between nonbondmg and n orb&&, mainly localized on the SCN, NC0 or NCS fragments The spectral mterpretatlon of CHsSCN IS based on semlemplrlcal CNDO/S calculations, sum-rule conslderatlons, and mtenslty differences between He(I) and He(I1) spectra For the larger molecules, comparison of the spectra 1s used as an aid m the interpretation In a number of aromatic lsocyanates (0-, m-, p-tolyhsocyanate and m-, p-chlorophenyllsocyanate), mteractlons between the isocyanate group and the highest occupied n and (5 orbltals of the phenyl ring are studied Spectra are assigned on the basis of semlemplrlcal INDO/S calculations

INTRODUCTION

The valence lomzatlon eneraes of some thlocyanates (RSCN, R = CH3, C2 H5, n-C4 H9 ), lsocyanates (RNCO, R = C2 H5, n-C4 H9 ) and lsothlocyanates (RNCS, R = C2H5, n-C4H, ) have been studled Mnth He(I) photoelectron spectroscopy CNDO/S calculations have been cmed out for the mterpretatlon of the spectra In the case of small molecules such as CH3 SCN, all the bands m the photoelectron spectrum can, m general, be assigned to lomzatlons from specific molecular orbltals For larger molecules, spectral overlap often prohibits a complete correlation between expenmental bands and lomzatlon processes A number of aromatlc lsocyanates have been studied (o-, m-, p-tolyhsocyanate and m-, p-chlorophenyhsocyanate), m which mteractlons between the lsocyanate group and the highest occupied a and u orbrtals of the phenyl rmg play a role Coqugatlve mteractlons and mductlve effects determine the energy differences between the orbltals and, correspondmgly, between the bands m the spectra Spectra are asslgned by compmson with semlempmcal INDO/S calculations

180 EXPERIMENTAL

The photoelectron spectra were recorded on a Vacuum Generators ESCA 3 usmg the He(I) line (584 a) as an excltatlon source The spectrometer, spectra were calibrated with the accurately known ionization enerses of the Ar doublet (15 76 and 15 94 eV) Lmeanty and scan mdth of the scannmg voltage were checked regularly The estimated accuracy m the position of narrow bands determined using this procedure 1s f 0 05 eV The resolution under the measunng condltlons was about 30 meV The He(I1) spectrum of methylthlocyanate was recorded on a Perkm-Elmer PS-18 spectrometer, using Ar and Xe as calibrating gases CNDO/S and INDO/S calculations [l-3] were carned out with a standard program, QCPE no. 144 In the calculation of the repulsion integrals TAB, use was made of the Panser-Parr [4, 51 or the Mataga-Nlshlmoto approxlmatlon [6] The molecular parameters for methyl- and ethylthlocyanate were taken from microwave data [7,8] The calculations for ethylthlocyanate were carned out for the non-planar conformation m which this molecule occurs m the gas phase [8] In the calculations for the other molecules, a planar structure of the alkyl part was assumed, using standard bond angles and distances

RESULTS

AND

DISCUSSION

Methyithrocyanate CNDO/S calculations Figure la shows the He(I)

photoelectron spectrum of methylthlocyanate Seven bands can be dlstmgmshed m the spectrum, the posltlons of which are gven m Table 1 Semlemplncal CNDO/S calculations (Table 1) mdlcate that eight orbltals are expected m the energy remon up to 21 eV The assignment of the first two bands 1s unambiguous The first band 1s sharp and charactenstlc of a nonbondmg orbital The calculations show that this band must be attnbuted to an a” orbltal, which 1s mainly localized on the sulphur atom with small contnbutlons from the carbon and nitrogen atoms This orbital can be regarded as the antlsymmetnc combmatlon of the out-of-plane sulphur lOne-pmr and TcN orbltals, m which combmatlon the fn-st strongly dominates The assignment of the first band to the a” orbital 1s also supported by a marked decrease m intensity m the He(I1) spectrum (see below) The second band 1s considerably broader, pomtmg to a more delocahzed orbital The calculations show that this band corresponds to an a’ orb&l, composed of the antlsymmetnc combmatlon of the m-plane sulphur lonepair and TcN orbltals The thud band shows a very intense adiabatic transltlon, followed by a

181

a)

CH,SCN

14 00

1600

d

7000

1600

1200

b)

1400

A

1000

1200

1400

7600

1600

-

1000 I

1200

1400

CH,SCN

1600

d)

1600

2000

2200

2400

n- C4H.SCN

16 00

1600

E (ov)

Fig 1 (a) He(I) and (b) He@) photoelectronspectra of methylthlocyanate, photoelectron spectra of (c) ethyl- and (d) n-butylthlocyanate

and He(I)

rather broad structure with an uTegular vlbratlonal progression The shape and mtenslty of this band suggest that it should be attnbuted to two iomzatlon processes The CNDO/S calculations also mdlcate a small energy dlfference between the third and fourth orbital Although CNDO/S results should m general be viewed with caution when orbltals of different symmetry are concerned, m thrs case both the order and the energy difference between the two highest occupied orbltals agree well urlth expenment The third orbital 1s an (I’ orb&l of the po type, mamly locahzed on the SCN group of the molecule with a large charge-density on the nitrogen atom The intense adiabatic transkon points to an orbital with conslderable lone-pm character, m agreement with the calcuhtrons The fourth orb&l 1s an a” orb&l which can be regarded as a symmetmc combmatlon of the out-of-plane sulphur lone-pm and WCN orbltals, zn which the latter dommates The fifth orbital 1s of the a’ type, composed of the m-plane sulphur and mtrogen lone-pan orbltals and, to a lesser extent, the TcN orbAaI The last three orbltals are delocahzed, mth rather large charge-density on the methyl group One of these orbltals has a” symmetry (methyl pseudo-r orbltal)

(13 13 15 15 17

6) 7 2 8 0

9 96 11 87 12 30

9 11 12 13 13 14 15 16 17

a’(CHs1

a"(CH3 )

a’W%1

a’(&N)

an(S, C, N)

a’(S) a’(N)

a”(S, C, N)

-12 -13 -14 -15 -16 -18 -19 -20 -27 -35 -38

08 54 80 18 44 36 22 09 46 77 62

IE

Type 77 63 64 15 6 2 3 0 0

IE

e,(CNDO/S)

Cz H5 SCN

-1198 -13 38

e,(CNDO/S)

a(C, N)

4s)

5Pe 9 64 1145 1195 12 6 13 3 13 9 14 4 15 2 16 5 17 0

IE -1199 -13 16

e, (CNDWS)

n-C4 )I, SCN

a”(S) a’(& C, N)

5Pe

N

CHJ SCN

z

EXPERIMENTAL IONIZATION EXPERIMENTAL IONIZATION ONLY TO 0 1-O 2 eV

ENERGIES AND CALCULATED ORBITAL ENERGIES (eV) FOR THE THIOCYANATES ENERGIES QUOTED TO 0 01 eV ARE ACCURATE TO *O 05 eV, THOSE QUOTED TO 0 1 eV

TABLE 1

183

On the basis of the CNDO/S calculations, the thud band (12 80 eV) 1s attnbuted to two lonlzatlon processes, 1 e from an a’ and an a” orbltal The fourth band (13 7 eV) corresponds to the fifth orbltal (a’), and the last three bands (15 2, 15 8 and 17 0 eV) are assigned to an a” and two a’ orbltals, the relative posltlons of which are uncertain Sum-rule conszderatrons The sum rule has proved to be a useful tool m the assignment of photoelectron spectra [9] In the composrte molecule approach, the molecular orbltals are composed of doubly occupied orbltals, localized on the fragments from which the molecule 1s thought to be built up For planar molecules the sum rule 1s valid for the partial sums of the a’ and a” orb&& separately If CHs SCN 1s assumed to be built up from CHJ , S and CN, these fragments contnbute through a doubly occupied orbital to the a” orbltal sum, leadmg to a complete cancellation of the mutual mductlve effects [lo] To the art orbital sum, CH3 adds 13 88 eV (derived from data on ethane [Ill), CN, 14 48eV (from cyanogen [12] ), and S, 10 4eV (first atomic loruzatlon energy), the total amountmg to 38 76 eV, which should equal the sum of the a” orbltals m CH3 SCN It 1s assumed that the fragments do not change their geometry on mcorporatron mto the molecule From the lonlzatlon energes of the x orbltals m CH3 CN, 12 46 and 15 7 eV respectively [13] , and the first atomic lonlzatlon energy of sulphur, a slmllar value of 38 56 eV 1s obtamed for the a” orbital sum One of the a” orbltals m CH3 SCN can be assigned unambiguously (loruzatlon energy 9 96 eV) The CNDO/S calculations indicate that the pseudo-x a” orbital should correspond to one of the last three bands m the spectrum (15 2, 15 8 and 17 0 eV) If the second a” orbital 1s attnbuted to one of the bands underlymg the thud feature m the spectrum (estimated lonlzatlon energy 13 0 eV), the sum rule locates the methyl pseudo-lr orbItal at 15 2 or 15 8 eV The assignment to the 15 8-eV posltlon 1s slightly preferred because of the somewhat better agreement urlth the sum rule and the CNDO/S calculations Changes m aniensrty m the He(I) and He(U) spectrum Changes m band mtenslty m the He(I) and He(I1) spectrum can be utilized m the assignment of photoelectron spectra [14-161 For the elements under conslderatlon, theory predicts that bands correspondmg to orbltals with mamly 2s or 3p character, relative to orbltals unth 2p character, should generally have a larger cross-se&on for He(I) than for He(II) radlatlon [ 171 The He(I1) spectrum of methylthlocyanate (Fig lb) shows a considerable decrease of the first band This confirms our assignment to a lone-pan orbital with domlnant sulphur 3p character In compounds such as CH, SH [16] and S(CN)2 1181 a slgruficant decrease m mtenslty 1s also found m the band which corresponds to this sulphur lone-pau orbital The mtenslty of the third band changes to a lesser degree The sharp peak on the low-lonlzatlon-energy side of the third band m the He(I) spectrum decreases m the He(I1) spectrum

184

with respect to the structure very close to it, m agreement unth our asslgnment to an a’ orbItal with dominant nitrogen lone-pan s character The centre of the structure as a whole shifts to a somewhat higher lomzatlon energy, caused by a small mcrease of the band which 1s correlated to a bondmg Q” orbltal, conslstmg predommantly of 2p orbltals on the carbon and nitrogen atoms The bands at 15 2 and 15 8 eV do not show clear changes m mtenslty relative to each other, since both the a’ orbital and the methyl pseudo-n orbital possess carbon 2s and 2p character An extra band m the He(II) spectrum IS vlslble at about 22 2 eV, which corresponds to an CL’ TABLE

2

EXPERIMENTAL IONIZATION ENERGIES GIES (eV) FOR THE ISOCYANATES AND IONIZATION ENERGIES QUOTED TO 0 THOSE QUOTED TO 0 1 eV ONLY TO 0 1-O

n-C4 Hg NC0

C2 H5 NC0 IE 10 10 13 13 14 15 16 17

32 86 2 9 8 5 6 8

e,(CNDO/S)

Type

IE

e,(CNDO/S)

SPe

-12 -13

a”(N, C, 0) a’(N)

10 14 10 62 118 12 2 12 8 13 6 13 9 14 6 15 3 16 6 17 7

-12 -13

a”(N, C, 0) a’(N)

16 26

C2 HS NCS IE 9 (9 12 13 13 14 16 17

12 5) 4 6 9 5 4 6

AND CALCULATED ORBITAL ENERISOTHIOCYANATES EXPERIMENTAL 01 eV ARE ACCURATE TO k0 05 eV, 2 eV

13 06

n-C4 Hg NCS %(CNDO/S)

‘l’me

-11

32

-11

72

a”( S, N) a’(% N)

IE 9 02 (9 2) 117 12 2 12 5 13 0 13 8 14 4 15 7 17 4

%(CNDO/S)

5pe

-1130

u”(S) N)

-11

a’(& N)

68

185

orbltal for which an orbltal energy of -27 CNDO/S The two lowest orbltals (CNDO/S be detected

46 eV has been calculated with -35 77 and -38.62 eV) cannot

Ethylthzocyanate and n-butylthrocyanate Figures lc and d show the He(I) photoelectron spectra of ethyl- and n-butylthlocyanate Expenmental lonlzatlon energies, CNDO/S orbital ener@es, and charactenzatlons of bands which can be asslgned unambiguously, are gxvenm Table 1 Ahphatrc lsocyanates and rsothwcyanates Figure 2 shows the photoelectron spectra of ethyl- and n-butyhsocyanate and of ethyl- and n-butyhsothlocyanate Phototelectron spectra of HNCO, CH,NCO, HNCS and CH3NCS have recently been pubhshed [ 19, 201, and several calculations have been carried out for HNCO and HNCS [ 21-241 Apart from the first two bands, the spectral overlap prohibits a complete correlation between experimental bands and ronrzatlon processes (Table 2). Al-

b)

a) C+l~NCO 800

mo

1200

1400

1600

1600

800

lOo0

12130

n C&NC0

1400

1600

4

I

800

J,

, 1000

_I, , 1200

I :

c)

,

,

1400

C,H,NCS

,

16 00

,

,

moo

,

-

Wg 2 He(I) photoelectron spectra ethyl- and (d) n-butyllsothlocyanate

-

600

I E (OV)

of (a)

ethyl-

and

(b)

Ar

hJ%+5J

i_! 1000

1600

, , cl)

n-C,&NCS

(

,

,

,

1600

1800

n-butybsocyanate,

and

1200

1400

,

of (c)

186

Aroma: trc rsocyanates Fgure 3 shows the photoelectron spectra of o-, m- andp-tolyhsocyanate as well as of m- and p-chlorophenyhsocyanate Vertical lornzatlon enerees and orbital energes, calculated with INDO/S, are gven m Table 3. For the bracketed bands m the table, no attempt 1s made to arnve at a one-to-one correspondence between expemmental and calculated lomzatlon energes

NC0

NC0

1,

800

1200

1000

I

I,,

1400

I,

1600

1800

800

loo0

1200

___f

800

12 00

1000

-

1400

1600

1400

I

1600

1800

E (eV)

1800

I E COV)

Fig 3 He(I) photoelectron spectra of (a) o-, (b) m-, and (c)p-tolyhsocyanate, m-, and (e) p-chlorophenyhsocyanate

and of(d)

95 112 114 116 12 6 12 9 13 4

90

IE

-12 -13 -13

51 36 37

-9 03 -9 67 -11 10 -1183 -12 27

%(INDO/S)

-6 65 -9 25 -10 69 -1173 -1199 -12 47

37 92 10 8 113 12 0 12 5

NC0

e,(INDO/S)

-4

IE

0

0

NC0

a’(Ph, N, C, 0) a”(Ph) o’(a) a”(N, 0) a’(N) o’(Ph) a”(C1) a’(Ph)

5Pe

12 7 13 1 13 4

88 97 11 1 11 3 11 5

IE

87 92 10 8 112 12 1 12 6

an(Ph, N, C, 0) o”(Ph) a’(N) a”(N, 0) a’(Ph) a’(Ph)

-12 -13 -13

69 26 42

-8 96 -9 80 -1110 -1172 -12 12

e,(INDO/S)

-8 71 -9 22 -10 31 -11 60 -1188 -12 43

QNDO/S)

CH3

NC0

IE

Type

00

NC0

-8 54 -9 43 -10 84 -1151 -1195 -12 31

86 94 10 9 11 1 11 9 12 6

a”(Ph, N, C, 0) a”(Ph) a’(N) a”(N, 0) a’(Ph) a’(Ph)

o”(Ph, N, C, 0) a”(Ph) a’( Cl) u”(N, 0) a’(N) a’(Ph) a’(Ph) a”(C1)

Type

E,(INDO/S)

IE

Type

00 3

NC0

ISOCY-

a”(Ph,N,C,O) a”(Ph) a’(N) a”(N, 0) a’(Ph) a’( Ph)

Type

EXPERIMENTAL IONIZATION ENERGIES AND CALCULATED ORBITAL ENERGIES (eV) FOR THE AROMATIC ANATES EXPERIMENTAL IONIZATION ENERGIES ARE ACCURATE TO +O 1 eV

TABLE 3

188

The agreement between the expemmental lomzatlon enerses and the calculated orbital energes 1s remarkable In the spectra of the tolyhsocyanates, s1x bands can be dlstmgulshed m the regon up to 13 eV These bands correspond to molecular orbltals composed of the highest occupied 7r and o orbltals m benzene and m the lsocyanate group In the spectra of m- and p-chlorophenyhsocyanate, two extra bands are present, which correspond to the m-plane and out-of-plane chlonne lone-pm orbltals The calculations mdlcate that the out-of-plane chlonne lone-pm orbital 1s stablhzed, and the m-plane chlonne lone-pm orbital ISdestabtized, by mteractlon respectively wlCh the highest occupied benzene a” (n) and Q’ (u) orbltals The mteractlon with the a’ and a” orbltals of the lsocyanate group is less important

REFERENCES

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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