Espectros Uv Vis n Alkanos

8/18/2019 Espectros Uv Vis n Alkanos

1/17

.IOI-KN \L OF MOLECI~LAK SPE~“I’IIOS~‘OPY

24, 2.32fi9 (1 ,(;7)

The Electronic Spectra of n-Alkanes t

The far Idtreviolel absorption spect ra of I he eight first Iiorrnsl paraffi~l hydra-

carbons were measured dower to 1150 allgstrcm~s with a McPherson model 225

II(lrmal incidence vacuum monorhromator completed with a model 665 douhlc

beam att arhment Imder approximately 0.2 A resollltion. Rlethane has a diffrlse

but not entirely strtlrtllreless I)and system with a masim~un at 1277 A. Accortl-

ing to its intensity it sho111d br relating ((1 an allowed transition and n111st. there-

fore be of the IF, - ‘~1 I t vpe. The c)t her seven molecldes all seem to have a weak

Iund near 1625-1575 .\. in is prohahly due IO a forbidden I.1

1 I.~I

transition

which was predicted by Mlllliketr in the case of et bane and probably has sirnil:

origins in all these molerrdes. Towards shorter wavelengths strong bands fol-

luw. The first strong balld system of ethane exhibits a relatively well-resolvetl

vihrat ional fine strllctllrr. The transition is very probably of the I. 3 + ‘_I, type.

The first and second strong bands (and probably the following ones) show a

systematic shift ioward longer wavelengths and a c*orrespo]ldillg increase ill

intensity. An attempt is made to interpret the spectra it1 terms of Mrdlikerr’s

lulited aiom treatment of the excited configllratiolls of methane and et hatIc>

and of the I’arisrr- and

Parr-f ype

calclllations of Katagiri and Satrdorfy.

INTI:OI)VCTION

iI

t,horough uuderstauding of the specb of aliphat,ic hydrocarbons is of funda-

mental importance for both molecular spectroscopy and quantum chemistrl. wt

I

t,hese ye&a received relatively lit,tle at,tention as compared, for example, to

aromat,ic molecules or coordination complexes. The reasons for this are undouht-

edly the difhculties encountjered in the theoretical handling of these g-electron

systems on t,he one hand

and

the instrumental difficulties involved lvith the

measurement of far ultraviolet absorption spectra on the other.

Among t,he more geueral recent, publications relating to aliphntic hydrowr-

~OIIS, IYC have t,o mention those of Okabe and Hcclwr (1) \vho mcasurcd thcl

0

spectra of met,haue, ethane, propane,

and rr-l)utnnc do\\-11 to 1250 A, I’artridgc~

(2) who invest,igat,ed the spectrum of polyet~hylenc, and Schoen (31 who measured

0

the spectra of et,hane, propane,

wld n-butane bct\veetl

1050 and X0 A. II(+h:111(’

* The research for this paper was sllpportrd in part t)y 1he Ikt’etlce I:mearcah Bmr~l of

Canada, grant No. 0530-63.


2/17

25-l

LOMUOS, SAU\.A(;EAU, ANl) SANlK)I:FT

and ethane received more &tention and n-e shall mention some of the previous

publications in t,he Discussion.

In t#his paper, \ve present the spect,ra of the first, eight normal paraffin hydro-

carbons in the gas or vapor phase.

We start, by describing our instrumental set,up which, we t*hink, makes it

possible to produce resultIs in the 2000-1150 A& art of t,he far ultraviolet8 with

greater efficient\- than has been hithert,o possible.

ESPECIMENTAL

Ai)

hTRUMENTAL

Full details of the apparatus \ve used will be given in a separat,e publicat,ion

(4). Here, we mention only the essential features of our setup.

Our measurements I\-ere made with a McPherson model 225 normal-incidence

vacuum moIlochromator. It was mounted wit’h a l-meter concave grating having

1200 lines/mm and blazed at 1200 ‘\ in t#he first order. The reciprocal dispersion

of this monochromator is ~3.3 X/mm.

The light source \\-a~ :I modified Tanaka-t)ype silica discharge Me which was

used with ultra high purity grade (gold label) hydrogen of the 1latheson Com-

pany (Fig. 1). The el&rodes, nlade from 1100-F ALCOA grade aluminium, \vere

consially placed in the discharge tube and \\-ere wat,er-cooled. The whole t,ube

\v:w again provided \\-ith a \vater-cooling jacket. Under t)hese circumst,ances we

were able t80 use m ac polver supply giving about one kilowatt (1000 volts and

1 ampPre) power to the discharge tube and we were able to use lo-micron slits

throughout’ the whole spectral area (a000 to lL50 _i). Like t’his our resolution

was approximately 0.2 angstrom. This represents 14 cm-’ at 1200 8, 10 cnrl

Y

at 1400 K, and S cnl-’ at 1600 A. The reproducibility of the emitted light in-

tensity \\-a~ better t8han f2.5’%.

The hydrogen discharge gives continuous emission down t,o about 1650 L\

but this is followed by a many-line spect~rum from l(i.50 to about 900 K. This

I4

I4

F’IG. 1. Modified Tam&a-type hydrogen discharge

t

rlhe. (2)

Conderrsatiou

1 he.

(3)

Cylindrical electrodes. (4) Tungstelt electrode connection. (5) Water in- and outlet. (6)

1’acllrm~ valve. 7) ater jacket. Over-all length 30 cm.


3/17

THE ELECTI:oNIC $Pl?CTRA OF n-ALKANICS

25,i

m&et: t he measurement of

nbsorpt,ion spectra difficult. In order t,o circumvent,

this difficulty, eve used a ,\IcI’herson model 665 double-beam

atjtachment which

became available recently for obtaining a continuous background. This double-

beam attachment has an oncillat8ing mirror arrangemeutj splitt,ing the hc:m~.

The t\vo beams fell onto t\vo sodium salycylatc coated plates \vhosc fluorescence

was dekected by a pair of EAII-ti25ti-S photomultiplier tubes. The t.w sign:&

were filtered wit’h a lwv pass filter and entered a pair of Keithley model 414

piw-ammet’ers for amplification. The two amplified signals \verc balanced iu :I

Burr and Bra\\-n log ratio converter (model IMT,), ;lrld the resulting sigrlal 1~:~s

fed into a Brist,ol recorder.

The minimum input sign:+1 level for normal operation of the log ratio converter

\vas verified to he 0.01 volt. Thr intensity of t,he emission of our hydrogen sourw

as measured directly never fell helo~v 0.15 volt. This applies not only to the emis-

sion peaks but) also t,c, the intensities in bet\\-een peaks \vhich arc usually* prc-

sumed t#o be zero. Thus, we \\-ere always sure of the transfer accuracy of the log

r&o converter. This is :L consequence of the high intensity of our source :md t hc

low dark current. level of our photomultipliers (1OP’ :m~pPre) aguilwtj an :~vcr:ug;r.

source intensity of lo-” amp&).

Since the t,ime const8ant of the Brist,ol recorder is 1 see for full-scale deflect8ion,

that, of t#he Iieit,hley amplifiers is bet,ter than 0.2 see,

and the response time of

the log ratio convert,er is at most 10-G set, the electronic resolut,ion is det’erminetl

by t,he recorder.

111

sing a scanning mte of 25 $‘min, a 0.-l-7-angstrom range of

t.he spect,rum passes through the slit per second. Like t.his, under t.he resolutjiou

of our monochromatjor, 0.2 A, the recorder can surely detect any variation (fine

struct8ure) of the same order. P’urthermore since t,he slowest scanning rate of the

monochromat,or is 0.5 X/min, any such structure can be easily verified.

The Lyman cr-line (1215.6 .i) and other hydrogen lines nere used for calibrating

t.he grating (5-Y’).

Our window material \vas Mgl~‘z supplied by the Opt,ovnc Company of Sort,h

Brookfield, hlassachusett’s. The limit that t’his matserial imposed

upon

our mew-

urement,s was 1150 A. It) gave a well reproducible, smooth background.

Lkhium fluoride windows are tjransmit8tant’ to about, 1060 Ai. We preferred

magnesium fluoride, however, because of the bettIer physical properties of the

latter material. In particular it is uonfluorescent (8) and it is insensitive to light,

and humidity.

Our spectra \vere measured under pressures varying between 1.5 and 0.25

mm Hg in a I-cm cell. The existence of the neaker hands \~ns checked by using

pressures up to 1 at#m.

R) THE SAMPLES

All the compounds we st.udied \vere oht,ained from the Phillips Petroleum Com-

pany of Bart’lesville, Oklahoma. Thew were of analytical reagent quality dr-


4/17

2x

LOPVIBOS, SAII\.AC;EAU, ANI) 8ANI)ORF~

livcred in small containers. Their purity is given by t’hc company as 99.99 molar

percent for methane, ethane, propane, and hexane, 99.93 for n-butjane, 99.92

for n-heptane, and 9931 for n-oct’ane. We did not try to purify them furt,her.

I’ully deuterated ethnne \vas supplied by A\lercl;, Sharp and Dohme of Canada

Ltd.

In none of0 our spect,ra did \ve register absorption at wavelengt,hs longer than

about 1600 ,4. This shoIvs t,hc absence of olephinic or other unsnfurat,ed impuri-

t,ies .

The spectra of hexane and heptane do not yuit,e fit’ into the series. This may be

at’ least, partly due to the presence of branched chain impurities which have

boiling point,s very close to theirs. Even if present in very small quantities these

impurit’ies, because of t’heir strong absorpt,ion, may cause t,he apparently diffuse

character of t,hese two spectra. (The spectra of a number of branched-chaitl

paraffins \verc measured in this 1,aborstor-y. See footnote ou the title page.)

The gas phase spectra of t#he first eight’ par&in hydrocarbons are sho\vn it1

Figs. 2

and 3. The locat’ions of t)he apparent hand maxima are collect,ed in Tables

I to VIII. They are given in angstroms, in cnrl,

:

md in elect’ron volts. The itl-

t’ensities are characterized by t,he absorption coeficient~s LY, n cm+ utm-’ and

the molecular extinction coefficient’s (t) in liters mole-’ cn~-‘. The nbsorptiou

cross sections (a) are also given (in mcgabarns).

545 595

635 680

725 770 815 860 CM-’

1835 1695 1575 1470

1379

1299 1227 1163 8

Fro. 2. Far 1111nviolet ahsorption spectra of methane, ethane, propane, and n-brttnne.

Rlolecular eslinctioll coefficients against wave nllmhers.


5/17

59.0 63.5 68.0 72 5 770 815

86.0 CM-’ X

A

i.i)

1./X

(cm-‘) 11 cew

01 (cm ’ t I mokl

atm-’ J

cm-l

1

CTMl,,

/;,, ,

(1425 )

70 175 s.700

12 119 0.5

(1390)

71 9-L” 8.019

80 782 3.0

(1315 I

71;190 I. Il(i

524 5007 1 ,.5

1277

is 278 ,.705

GO0 583) 22,, 3

0.2ti

(1 Itil’- )

86 029 10 fjli5

iilf) liO20 “:I.0


6/17

2.x

LOMBOS, SAI;\‘A(:EAU, ANI> SANIWRFY

TABLE II

AHSORPTION PEXS IN THE ELECTROKIC SPEC’TRUM OF ETH~NE

C = 8.02 X 10e5 imole 1-l). L = 1.00 cm,

7’ = 295X, p = 1.5mmHg

l/X (cm-‘) IC (eV)

a (cm-l

e (1.mole-L

atm-*) cm-l)

v (Mb) J.,,L.

(1575)

(1525)

(1490)

T’, : 1423.0

1’2 : 1401 .li

l’:, : 1380.0

1-4 : 1358.8

TY, : 1339.1

1’6 : 1318.6

lTi : 1297.1

1’8 : 1277.3

1’9 : 1257.3

r’lo : 1240.1

V,, : 1221.3

(1180)

(1150)

G3 49-l 7.872

65 574

8.130

67 11-l 8.321

50 269 8.712

71 347

8.845

72 464 8.98-l

73 594 9.124

74 ( 77

9 ,706

2

C X 1O-5 L T p

E (eV)

E (1

.

mole-’

cm-l)

g (Mb)

j,,, I,

(1590)

62 893 7.797 37 360 1.4

(1577) 63 391

7.859

55

539

2.1

(1512) 66 116 8.197

169

1 641 6.3

1395

71 685 8.887

1155 11 240 43.0 0.33

1285

77 821

9.G48 1338

13 010 49.0

1200 83 333

10.331 1513 14 720 56.3

1172

85 288

10.574 1592

15 480 59.2

(11621)

86 022 10.665 1546 15 040

57.5

can be cnlculat,ed only approximately and that only in a few cases. For hexane,

heptane, and octane the low pressures which were needed made it difJicult8 to

measure the pressure accurat8ely and t,he differences in the j-values for these

compounds are not, considered significant.

Dikhburn (IS) gives about 1s 1Ib for the cross section of the 127-i-Xand of

methane. Sun and Weissler (I.?) give 20, AJoe and Duncan (23) 23, and Watanabe

(21) gives 19. Our value is 22.3. For the oscillatjor strengt,h of t,he same baud ;\ioe

and Duncan (23) gave J’ = 0342; Wilkinson and .Johnston (22) e&natcd 0.1.

We obtained 0.26.


7/17

THE EI,F:C’TI:ONIC SPECTRA OF n-ALKANIP

2.X

TABLE I\.

ABSORPTION PJLUCS N THE ELECTRONIC SPECTRC’M OF WBUT.\NE

C’ = 3.21 X IO-” (mole l-l),

L = 1.00 cm,

7’ = 295X, p = 0.6 mm Hg

x A,

l/h (cm-‘)

/: (eV)

01 (cm-l

t (I.mokL

atm-‘)

cm-l)

0 (Mb)

.i,s,n

(IGOO)

ti2 500

115i5)

G3 492

(1550)

64 516

Cl520 j 65 78Y

1410

70 ()‘W__

1335 74 906

(1237) 80 808

(1202, 83 Iti0

Illi

86 207

(11501 i

86 957

i.749

2x 2W

1.0

7.872 104 1010

3.9

i.999

104

1010

3.9

8.156

298

289G 11.1

8.793

1586

15420 59.0 0.5

9.287 1845

17950 68.7

10.01s

2136

20780 79.5

10.310

2233

2 1720

83.1

10.088

2531 24620

94.2

10.781 2385

23200

88.8

TABLE \.

ABSORPTION PE.\ks IN THE ELE(TRO~YI(~ SIWTRCZI ok 12-I ENT.INE

C = 2.67 X 10-j (mole 1-I).

I, = 1.00 cm.

y = 295aK,

p = 0.5 mm Hg

x (5,

l/X (cm-‘) I( (ev)

01 cm-’

t (1 moleP

atm-I) cl+ I

(r (MIJ)

/ ,,/n

1GlOJ

G2 112 i.iOO

96 J31 3.

fi

11575)

(3 492 7.872

133 1 290 5.0

1415

i0 (ii1 8.iCi_

1781 li 330 Mi.3 O.Ci

1345 74 349 9.218

2094 20 300

77.0

11170:) 85 470 10.591i

2909 28 ROU 108.3

TABLE \‘I

ABSURPTIO~ PE.\KS IN THE ELEWRCJNIC , jPE( TRlIM F TZ-IIEx~NE

( = 1.87 X lo-” (mole l-l),

L = 1.00 ?“I,

T = 295x,

p = 0.35 mm Iig

l/X (cm-‘J

1: eVi

t (I mokl

cm-’ I

(lf22

1

01 728

(1570)

63 694

(1520)

65

i89

(1475

I

67 797

(l-I-15)

69 204

1125

70 1175

1335

74 9oti

(1230

i

81 301

c 190 )

84 034

tllti5r) 85 837

7. ti5:<

7.897

8.156

8.405

8.580

8.700

I. 287

10.079

10.418

10.G42

141

1 3x 5 .3

323

3 145

12.0

808

7 8ti2

30.1

1500 14 590

55.8

1849

li 980 68.8

2010 19 560

74.8

(0.9)

2475 24 080 92.1

2894 28 1w 107.5

3258 31 ti90

121.3

353 1 34 350 131.4


8/17

LOMB08, SAU\‘AGEAU, ANI) SASI)ORFY

TABLE VII

ABSORPTION PEXS IN THE ELECTRONIC SPECTRUM OF IL-HEPT.\NE

C’ = 1.61 X 1O-5 (mole I-‘), I, = 1.00 cm,

7’ = 295X, p = 0.3 mm Hg

x

(8,

l/X (cm-‘)

Ii (eV)

IY cm-l

t (1.mole-’

atm-‘) cm-l)

(r (Mh)

./,jf L

(lG25j 61 538

7 l i 29

145

1 410 5.4

(1570) 63 694

i

,897 331

3 223 12.3

(1515) 66 007

8.183

911 8 863

33.9

(1475) 67 79i 8.405

1532

14 910

57.0

1430 69 930 8.669 2154

20 950 80.2

(1 .O)

1340 74 627 9.252 26il 25 980

99.4

(1240) 80 645 9.998 3120 30 350 116.1

1220 81 967 10.162 3244

31 560

120.7

(1200) 83 333

10.331 3430 33 3io

127 7

(1ltiOT) 86 207 10.688 3551 37 470

143.4

TABLE VIII

ABYORPTIOS PE.UGS IS THE ELECTRONIC SPECTRUM OF ~-OCTISE

C =

1.4 X 1O-5 (mole l-l),

I, = 1.00 cm, 2’ = 295”K, p = 0.26 rnIl1 Ilg

x

(4

l/X (cm-l)

(1630) (il 350

7.606 119

1 158

(1580)

63 291 7.847 111 1 081

1550

64 51fi

7.998

341

3 320

(1530)

65 789

8.156

429 4 170

1415 70 671 8.762 2524 24 550

(1395)

71 685

8.887 2492 24 250

(1380)

72 464

8.984

2556 24 860

1340

74 627 9.252

2977

28 960

(1285)

77 821

9.648

2977

28 960

(1245) 80 321 9.958 3254 31 6G0

(1215) 82 305 10.204 3413 33 200

(1185) 84 388

10.462

4151 40 380

(1175) 85 106 10.551 4223 41 080

(11501)

86 957 10.781

4048 39 380

Ii (CV)

01 (cm-’

atm-I)

E (1. mole-’

cm-‘)

rJ (Mb)

jr,,

,

___

4.4

4.1

12.i

16.0

93.9

(1.2)

92.8

95.1

110.8

110.8

121.1

127.0

154.5

157.2

150.7

There are one or tww weak bands or at least, a pronounced idkctkJll het,\vcell

1630 and 1575 .k in all the spectra except the one of methane in which a similal

feature is found near 14“:’ K. Then follo\v two broad bands representing in ~11

probability two diffuse band syst,ems lvith fairly well-defined maxima. Only in

the case of ethane is any fine structure resolved. There are evidently further

band syst’ems at. higher frequencies but’ their maxima are even more dificult, to

locate.

Table IX shows the trends in frequencies and int,ensities for t,he first two


9/17

THE ELECTI:ONIC SPECTKA OF n-,4I,K.4NES

strong, diffuse band systems. The weak bands at t,he foot of the first st(rong band

shift’ gradually toward longer waves for the four or five first, numbers of the

series at least. This shift, is large between ethane and propane (about, ltiO0 cn~-*

) ;

t,hen it reduces to a few hundred cm-1 or less between successive members of thcb

series.

The t \vo strong ones first exhibit a much larger shift (about, 430 cm-’ bet\vcwl

ethanc :u~d propane tcnvard lou-er frequencies converging (for the first OIIC) to ;I.

limit at nboutj 1#20 L\ nhich they reach for pentattr. There CWII seem to tw :I

t

rndency t,o reverse t,he direction of shifting for woctane. :2t t,he same

t

imc

t hc

intensit,ies increase gradually and seem to attain a plateau. However more accw

rate mcasurement~s of the pressure will he needed before this

can he

ascertained.

The spacings in t,he vibrational fine structure of cthane are given in Tahlc S.

li,levcw pe:ks :u’e clearly distinguished. The spacing is tlot quite regular. ITor


10/17

262

LOMR08, SAW-AGEAIT, AND S,4Nl)OI:FY

TABLE XI

ABSORPTION PE.\KS IN THE ELECTRONIC SPECTRUM OB ETH~NE-d6

C = 7.48 X 10m5 (mole l-l),

L = 1.0 cm,

?’ = 295x, p = 1.4 mm IIg

(1562) 6-l 000

(1517)

65 898

(1482)

67 454

T-1 : 1392.5 71 813

1’2 : 1376.9

72 627

v, : 1360.0

73 529

1-a

:

1343.8 74 416

T-i, : 1327.5 75 330

T-6 : 1312.5

ici 190

T’i : 1295.6

77 184

Iv8

:

1880.0

78 125

1’9 : 1267.8 78 877

I-10 : 1251.9

79 879

T?n

:

1237.5

80 808

llti3.7

85 933

(1150)

86 957

l/X (cm-‘) R (eV)

7.93G

8.170

8.363

8.903

9.004

9.116

9 226.

9.339

9.446

9 .509

9.686

9.779

9.903

10.018

10.654

10.781

a

(cm-’

atmel)

e (1.

mole-’

cm-l)

21

20 0.8

31

303 1.2

39

376 1.4

If

1630 6.2

230

2239 8.6

358

3482 13.3

475 4623 17.7

593

5764 22.1

67ti

6573 25.1

713

6934 26.5

731 7 1 0 7 2 7 . 2

696

( i i 7 5 2 5 . 9

Gtil

M’S 2l.G

ti36

6183 23. i

ti50 6327 2 4 . 2

590

5735 21.9

t he first, members it is about1 1100 cnl-l; then it alternates, wit h about, 1150 cm-l

and 1250 cm-l.

Tables X and XI cont,ain t,he data pertaining t’o et,haw.A6 .

14.

I\IETHrlNE

A4mong all sat,urated hydrocarbons mctShane received by far the most’ at,tention.

The most complete \vorlts seem to be those of Dikhburn (I.$) and of Sun and

Weiwler (15). These authors and Weissler (16) reviewed the earlier literature

I

(9, 10, z~-z?O)

and we believe t’hat there is no need of doing t,his again.

hll previous authors agree tIhat8 t,here is a diffuse, structureless band centered

at about 1277 Lk (7s 2SO cnl-I) (our data). This is folio\\-ed by much stronger

absorption toward shorter wavelengths reaching a maximum near BfiO .I and

then decreasing gradually up to 3;iO .& (J, 11, I./t). What we obtained is merely

the longest, wavelength part of this spectrum. It is considered to bc a sep:lratjc

electronic

hmd

syst,cm. The plat,eau immediat8ely follo\\ing the shallow peak

at 1277 .i in our spectrum is also clearly distinguishable in the spectrum COII-

tained in Weissler’s rcvitw (16).

The diffuswless of the spectrum of methane \vas uttjrihuted by Price (9) and

by Duncan and Howe (19) to dissociatjion or predissocintion in t,he excited st,:ites

and t,his view is shared by all later authors. [Ionization starts at, higher fre-

clucncics only (1-C) l-i,

,“I ). I


11/17

h point, t,hat, deserves discussion is the :qq~arance of :L \veak branch at the

longer wavelengt’h side of this band. It is ~41 dist~inguishnhle in t,hc spectrum

given by Wilkinson and .Johnston (22) and 1\I0e and I~unc:m (53) and is a1s0

present’ in the one by Sun and Weissler (15) and in our OWI met,hane spectrum.

(We hnw verified its presence by using l&m prcssuw.)

We should like to raise the quest,ion if this branch could correspond to :t stpa-

rate forbidden transition. This possibility would arise if the first, escitetl state*

resulted from the jump of an electron from a triply degenerak J.Lorbital of the

ground state to ml cscited ,fi2orbital. Then the excited st,ate I\-ould split according

to F: X F? = 111 + I:’ + Fl + F: , t,he transition to F, being :4o\ved (from the

A1 pourd stat’e) and the others forbidden. The latter ma\- be made allo\ved b\-

nontotally symmetrical vibrat,ions of suitable symmetry (/\) which arc :~v:tilablc~.

The exist,cncc of these transitions even t,hough they are expected to be \\wk ma ’

also contribute to blurring t,hc observed hands and one of thcJn nxty relat,c* to the

NC& branch at its long \\.avclength side.’

According to calculations of Iktagiri arid Sandorfy (l.i?) \\-ho ttpplictl t)hc

l’ariscr :t~rtl Parr method to sat,urat,ed hydrocarbons, the first, tr:rnsitBions would

result from :~rl al + J> jump not ji + $? . This would make the first band

F2 c- .A

:~ntl :~llo\~c~l. The order of the states, however, depends in :L delicat#e way on the

choiw c f cert,ain paramct’ers and a reversal appears quite possible.

AIulliken, in 19X.5, examined the excited configurations of methane in t,erm:: of

united atom orbitals (Z$). (See also Reference 87 for :I more recent confirmatioIr

of his vic\vs.) According to him,

“As lowest excited orbitals of CrH, , it seems

r(woIlablc to expect Rydberg, i.e., large atom-like orbit& 3s, 3p, 3rl, et,c. with

1%~o\\rst,.” Then the first transition would he (omitting carbon Is electrons),

I~aJ~[r ~~]“(;~.sal), F, -

[s~~]Ypj~]~~,4 1

This is essentially of tbe 3s + 21) t,ype :trld

al11NXY~. It is not’ ahsolut8ely necessary,

ho\vever, t,o consider these orbitnls :LS

R\dbcrg orbitals. As 3lulliken point,ed out in the same :trt,icle, ant,ibonding

localizc~tl (‘-- H orbit& are “qualit,ntively veq

similar to t#hose of 3s and :


12/17

“6-l

LOMBOS, BAUVAGEAU, AND SANlXJllFY

R ETHAXE

The appearance of vi$ational fine structure in the hand syst’em of ethane

cent,ered at, about, 181s X (75 S-IO cm-‘) is a highly significant fact because it

shows that, t#he lo\\-er exited states of normal-saturated hydrocarbons are not

necessarily repulsive or highly predissociat8ed as it, is oft,en presumed. (That is,

the lifet,ime of tbe excited state is not so short as t)o cause t’he obliterat8ion of all

vibrational fine structure.)

The spacing between t,he observed vibrational bands is approximat’ely 1100

to 1250 cm-l. This is in agreement with results communicated by Raymonda

and Simpson (27) who found it about, 11.50 cm-‘.

Under D:,,, (eclipsed) or 11:1,, (st#aggered) symmetry ethane has only three

totally symmetrical vibrat8ions. One of these, a CH strekhing motion (1’1)7 has a

high frequency, 2899 cm-’ in the ground state. IT2, the sgmmet,rical CH bending

vibration, is at, 1375, and I’:< , the C-C stretching, at, 993. all Raman active

(2%). An excited-state vibration of 1150 cm-l might well be the I-, vibration of

the excit#ed state but, it is not, impossible that it is V, whose frequency would in-

crease if t#he electron was taken from an orbital which is ant$ibonding in t,he C-C

link in t,he ground stat,e. In order t,o find the ans\ver to t#his question ne measured

the spectrum of ethane-t/e (Fig. 4) alt,hough we had only a slight, hope t#hat, it,

could provide it. 112 has the frequency 1375 for CzHs and 11X for C2Ds in the

ground state, giving a rat’io l.lS7; IT3 is at 993 for CzH6 and X2 for C&DE , giving

ETHANE (1)

100

ETHANE - D6 (2)

90

80 -

70

60

50 -

40 -

30 -

I >

500 545

2000 (835

590 635 680

725

770 8t5 860 CM’

1695 1575 (470 1379 1299 (227 63 %


13/17

a ratio 1.165; furthermore according to the normal coordinat,e calculati(Jlls of

Schachschneider and Snyder (291 the t,wo modes are highly mixed for the dew

terakd specks.

I,ooking more closely at the frequency values of the individual peaks \vc find

fairly serious deviat,iona from the 1 l.‘,O-cm-’ value and :IJI alternating scheme

seems to manifest itself (Table XI).

The average of t,he ratios of t,he corresponding vihrat,ional frequencies fob

C’?Hs and c-)2D6 is about 11.29. Thus the deuterated compound did provide at

least a partial answer to our problem. The high value of the ratios clearly in-

dicates that t.he vibration \\-hich appears in the fine structure of the spectrum of

cthane is :I, C--H vibration. It is surprising, however, that Jr2 in the relating

excited state of C2D6 is mixed \vith C--C motion to a much lesser extent than

in the ground stat.e and we cannot he sure t,hat t.he vibration which is involvc~tl

is actually l’, . It, may be a consequence of the distortion of the molecule in the

excited state. The blue shift, of t,he \vhole band syst,cm is probably connected

1yjt.h this too and indicates strong vibronic int~eruct,ions.

I,onguet-Higgins, ijpik, I-‘r\.ce,

and Sack (30) published an cst,ensive trw-

nlent of tbe vibronic structure of E - d type transitions where t,hc 1’3 tate hae

one doubly degenerate vibrat.ion. This is probably close to the case of this b:u~l

system of et.hane. They predicted a complicated and some\vhat irregular vihrn-

t,ional patt,ern wit,h two maxima. All this seems to he iu clualitative agreement

\\-ith our spectrum. As to the wcoud maximum \\hich is at about, 1210 At (SO til0

cnl-1) this may he boosted by overlap from the nest higher band y&em (which

is diffuse wit,h some structure) but, this is likely to he only l part8ial cause of t’lw

:~ppe:~rance of tnu maxima.

If higher resolut,ion \\-ark confirms this interpretation then ethxne may ~WJIW

arl example of t,he .JahwTcller effect, in the gas ph:w.

In his above cit,ed paper AIulliken (24) discussed t’hc lo\vw excited configura-

tions of ethane. He used CHa group orbit& to describe the ground state except

for the (‘--C bond in which t\vo 2~1a orhitals overlap giving a [D + C] bond orhit:

:

[sa [SUl]~ w]” lar]’ [u + (T. a$. ‘=I1 .

C’H:, C’H:j C’H:< ‘H:{ C’-- C’

‘rhc lcJ\vCst, excited orbitul ww taken by 1Iullike~l to be wsc~rltiall\~ of th(l :is

1~~J~e; . . .

ve

might first think of a :


14/17

260

LOMBOS, SAlJ\‘AC;EAlT, ANI) SANIXIRFY

The first allowed transition nray be due to a kmsition from ~1 degenerate

x-t,ypc orhit8al \vhich is implied in the C:Hg-bonds in

any of

the CH, radicals t’o

the 13s + &s] orbit,& This transition would be of type E + A1 and allowed

and probably strong since it, resembles a 3s

+- 2p7r transition of an atom.

According t,o I\at,agiri and Sandorfy (12) t,he first transition of staggered ethanc

Jvould be h’,, + .J1, allowed and polarized perpendicularly to the C--C direction.

The next one would bc ISfl +- A Ig (forbidden). It folknvs a transit’ion between

two doubly degenerate orbit8als (e, t,o e,,) with t,he excited state split, into -41~~+

rl ,‘ + EC, )

t,he transition t,o Ahc-being forbidden, the one to A: allowed and

polarized according to the C-PC direct,ion,

and the one to E,, perpendicular to it,.

The situation is similar for eclipsed ethane but only A ?” \vould be allolved for

t.he lat.t.er transition.

As in the case of met#hane the order of the transitions may chungc if somewhat

different paramet.ers or if higher approximations are used. Should the first t,ransi-

tions t,ake place bet,wecn states resulting from a jump bet,\veen t\vo degenerate

orbitals there could be again weak electronically forbidden transitions giving

bands at. the lowfrequenc~ side of t,he first strong band system and this could

be another possible e~planntion of the \venlc bands near ltiO0 x. This inkrpreta-

tion is made unlikely, ho\vevcr, by the fact that the e,,

+ c, t.ransit8ion would give

rise to states much higher than the I{,, + A,,, transition (12).

According t,o both 3Iulliken and Katagiri and Sandorfy t,he electron making

the jump giving rise to the first8 skong absorption band depart#s from t,he C-H

bonds and the transit,ion is of t,he F +- A t.ype. Tn all probability t,here is a third

band system at aboutS 1200 8. Furt~her spectroscopic and theoretical \vork is

needed before more definite assignments can be made. As for the latter we hc-

lieve t#hat our remarks relaCng to met,hane apply t#o the case of et,hanc too.

c . THE X~EMAI, I AHAFFINs FROM P~oP~4~ac TO OcTAxE:

There is a pronounced red shift, from et,hane to propatit for the st,rong bands

and t,here are probably three or four electronic hand systems in t#he spectrum of

the latter. E’urt*her lengthening of t’he chain produces only lesser changes in t#he

frequencies (Table IX, l’igs. 2 and 3). There are several electronic transitions in

all these spectra which are distinguishable at variable degrees according to over-

lap conditions for the individual molecules.

Okabe and Becker (2) measured these spectra from methane t,o n-butane.

There is general agrecment~ bet#ween t’heir specka and ours inasmuch as the areas

of absorption are cc;icerncd. However, since their resolut,ion was only about 1 Lk

while ours w-as 0.2 A, our spectra are more detailed t)han t,heirs.

Kat,agiri and Sandorfy (12) gave values for the six or seven lowest elect’ronic

energy levels of propane. Except the first, one they lie close to one ot.her and it. is

not surprising therefore that complicat’ed spect,ra arise.

So vibrational st~ruct,urc is present, in our specka. This does not mean, however,


15/17

in our opinion, t,hat, disso&t,ion or predissoci:~tion art thr onl - c:uwes of the

breadth of t,hese bonds.

Other possible causes are the existence of man\- close-lying

dectr J ni c states,

the

increasing number of rotational isomers (it \vas shown in Reference 12 that

there may he significant differrnces in the spectra of rotational isomers), ant1 the

increasing number of totally symmrtric:tl vihwtions drw to the ion- symmetr\~

of these molcculcs and their size.

It is often presumed that in saturat’ed hydrocarbons the CkCtrcJlIS are lr,calized

in one given C---C or C’--H bond. This is approximately true in t,he ground state

although it is n-e11 kno\vn that bot,h the heats of formation (21, ,3/, 32) and the

ionization potentials (33, ,34) shO\V a definite trend indicat8ing :L certain degree of

c~clocalizat,ion. The compurisoll of our spectra illust.rates t.hc fact thi\t such

localization is certainly not admissible for the excited states.

The energ). of any photon lvhich can bring ahout an electronic (singlet-singlet

)

transition in a saturated hydrocarbon would be suficie~~t to dissoci:~tr an:,

given bond in these molecules. However, if there is enough int,eractioll bct\veell

I

he bonds in the excited state the excitation energy will hc divided het,n.ecw them

precluding to dissociation. Similar argument’s \rere put forward b3‘ Mugee rt ul.

(.36, Jfj) relating to infinite chains and l’artridge (2) :ipplied them to both J)o]).-

ethylene and dkarrcs.

There are Iveal< bands betwcn l(i:l

of most of t,he other bands entirely to dissociation or predissociation. This band

y,.stem of c~thanc may be XII cwmple of the .Jahrl-Teller effect. in the gas ph:w.


16/17

2%

LOMBOS, S4U\-AGEAU, ANI) SANI)OI:FY

The use of double-beam inst,rumenbs in t,he measuring of far ultraviolet ab-

sorption spectra opens the way to relatively rapid progress in the study of t(he

tBransitions of g elect8rons and other spectra located in the far ultraviolet.

Our very sincere ttra~lks are d11e to I)r. Yoshio Tanaka who allowed one of US (B. A. L.)

to spend :L day in his Laboratory at 1 e Air Force Cambridge Research Laboratories in Bed-

ford, Massachtlsetts providing US with mrxh good advice concerning discharge tttbes and

other far ultraviolet problems. We are also indebt,ed to 1)r. 1:. Ii:. Huffman from the same

Lahoratory for valtiahle help at the same occasion. I)r. F. R. Lipsett from the R.adio and

EXectrical Engineering 1)ivision of

the

National Research Council of Canada, Ottawa also

gave US the benefit of his far Illt raviolet experience at the start of

this work.

We express ollr

sincere thanks IO him. We acknowledge a stimulating conversaiicm with Professor T. M.

I)r~nn from the ITniversity of Michigan. We thank Professor Marcel Itinfret, from our de-

partment. and Mr. J. Basinski and Mr. 0. Schwelb from t,he Northern Electric Company of

Ottawa for helpfIll discrlssions. We express olr appreciat,iorl to Mr. George Sarfi from

“Qrlart z Works. Precisiotl (;lass Blowing Co.”

of Montreal and to electronicians and tech-

liicialls Mr. J. L. Lepage, Mr. LLIcien Chiasscm

, and Mr. Frangois Roy. We gratefrlllg ac-

knowledge finanrial help from the National ltesearch Cormcil of C xnad:t.

ItECEIVEI : l~ehruar?_ 16, 1967

I. H. OKABE AND 1). A. BKVKIG:II, J. C’hetn. Phya. 39, 2549 (1963)

S. 1:. H. PARTIlIDoE, .J. Chew Ph.//s. 46, 1685 (1960).

S. 1:. I. S(:HOEN, I. Charn. Ph ys. 37, 2032 (1962~.

/f, B. -4. LOMBOSAND I-‘. SAI’VAGEAT- irk press).

5. B. JAY, I


17/17

THE l~~LECTI:ONIC SI’ECTl:A OF n-ALKANES % I

25. Ii. S. M~ILLIKEK, J. .~JH.

(‘htvtr. Sot.

86, 3183 (19G-l).

,Ofi.

W. T.

SIMPSON,

“Theories of

I~lertror~s lr

Wlolertdes.”

Prelltice-Hall, Inc., ICnylewooti

Cliff’s, Sew Jersey, lS(ilZ.

??. .f W. I~AYMONDA AND W. T. ~~I.MPHoN, in ,

Espectros Uv Vis n Alkanos

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