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.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.
8/18/2019 Espectros Uv Vis n Alkanos
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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.
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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-
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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.
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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
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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.
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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
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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
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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
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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
8/18/2019 Espectros Uv Vis n Alkanos
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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 :
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“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 %
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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 :
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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,
8/18/2019 Espectros Uv Vis n Alkanos
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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.
8/18/2019 Espectros Uv Vis n Alkanos
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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
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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 ,