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The microwave spectra of symmetric top polyacetylenes
JOURNAL OFMOLECULAR SPECTROSCOPY 70, 84-94 (1978)
The Microwave 1,3,5-Heptatriyne,
Spectra
CH&=C)3H
of Symmetric
Top Polyacetylenes
and I-Cyano-2,4-pentadiyne,
CH,(C=C),CN
A. J. ALEXANDER, H. W. KROTO, M. MAIER, AND D. R. M. WALTON School of Moleculsr
Sciences,
University of Sussex, Brighton BNl
9QJ, United Kingdom
The rotational spectra of 1,3,%heptatriyne, CHa(CrC)aH and l-cyano-2,4-pentadiyne, CH)(C=C)ZCN have been studied in detail between 26.5 and 40.0 GHz. The molecules have
long linear chains of heavy atoms and show characteristic Ca, symmetric top spectra consisting of groups of R-branch lines at regular intervals separated by approximately 2Bo. Six isotopic modifications of CHr(C-C)&N have been detected in natural abundance allowing r, substitution structural data to be derived for this species. Long linear polyacetylenic chains are quite flexible and this dynamic property manifests itself in the appearance of extended sequences of complex vibrational satellites associated with the bending of the chain. The vibrational ground state spectra as well as several low frequency vibrational satellites have been analyzed yielding various vibration-rotation parameters. For CHs(CrC)3H BO = 778.2445 f 0.0007 and for CH,(CsC)XN Bo = 778.040 f 0.001 MHz. INTRODUCTION
In a previous study (1) an accurate analysis of the microwave spectrum of the long linear polyalkyne (polyacetylene) cyanobutadiyne, H(C=C)KN, was carried out. This molecule was of interest because it was the longest linear molecule thus far synthesized and showed interesting structural and dynamic properties associated with extended chains of atoms (2). Furthermore, on the basis of these laboratory measurements it has been possible to identify this species in the interstellar medium by radio astronomy (3). Since this work, we have synthesized the next member, cyanohexatriyne, H(CaC)3CN, (4) and also detected it in interstellar space (5). These species are the most complex interstellar species so far detected and are significant in that this complexity must be accounted for by any general theories of interstellar molecule formation. In this study we present the results of an extension of these techniques to the synthesis and study, by microwave spectroscopy, of two new CIV symmetric top polyalkynes: 1,3,5heptatriyne, CHI(CaC)BH, and 1-cyano-2,4-pentadiyne, CHI(C~C)GN. Microwave techniques are ideally suited to the study of such polyalkynes because under the low pressures required (- 10 pm Hg) the polymerization rate is slow and the molecules are stable for several hours. In the condensed phase at room temperature they polymerize rapidly. The long linear chains are quite flexible and the consequences of this in the spectra are of interest in that a large number of vibrational satellites occur, in fact, they domi84 OO22-2852/78/0701-0084$O2OO/O Copyright AU rights
@
1978
by Academic
of reproduction
in any
Press, form
Inc. reserved.
SYMMETRIC
TOP POLYACETYLENES
85 u^ “0 7
-21
1
d 0 2
86
ALEXANDER
ET AL.
nate the spectra. It has proven possible to analyze part of this structure and derive associated vibration-rotation parameters. Some of the isotopic satellites of CH,(CG)&N have been detected in natural abundance allowing substitution structure data to be derived. This has permitted the effects of chain lengthening on structure to be compared with similar effects in
:Hz
se:8
39.2
39.4
J=25c24 0
?HZ
I 36.4 b)
I
CHS-GC-CS-
I
I
36.2 C=N
J- 23t.22
I
I
I
36.0
I
35.3
1
FIG. 2. Medium resolution scans of the structure associated with a single I transition. CH,(CnC)aH (a) and CHa(CkC)KN (b). The lines of the latter are somewhat broader and considerably more intense. The bending vibrational satellites tend to lie in groups with the same value of II, and march out to high frequency as indicated. Same conditions as Fig. 1.
SYMMETRIC
TOP
POLYACETYLENES
TABLE Ground
1.395 k
J
17
28016.57 28016.45 28015.92 28015.15 28014.02 28012.58 28010.88
J 0
1 2 3 4 5 6 7 8 9
19 -0.02 0.09 0.03 -0.04 -0.04 0.01 0.01
31129.47 31129.30 31128.72 31127.88 31126.62 31125.05 31123.10
35798.79
-0.00
35798.62 35797.98 35796.95 35795.55 35793.71 35791.42 35788.81 35785.72 35782.20
0.03 0.00 -0.01 0.02 0.02 -0.03 0.02 -0.01 -0.06
37355.22 37355.00 37354.40 37353.30 37351.84 37349.91 37347.55 37344.77 37341.62 37337.95
29573.02 29572.96 29572.39 29571.47 29570.29 29568.62 29566.97
-0.01 0.03 -0.01 0.01 0.01 0.00 0.00 -0.00
22
0 1 2 3 4 5 6
28009.19 28009.04 28008.57 28007.82 28006.67 28005.30 28003.56 J
0
1 2 3 4 5 6
-0.04 -0.03 -0.03 0.01 -0.03 0.03 0.04
29565.29 29565.10 29564.60 29563.76 29562.61 29561.06 29559.22
0.03 0.03 0.01 0.01 0.00 0.02 0.01
35789.38 35789.20 35788.60 35787.58 35786.15 35784.33 35782.09
21
34233.41 34233.21 34232.62 34231.64 34230.28 34228.55 34226.41
32685.91 32685.78 32685.20 32684.25 32682.95 32681.28 32679.20 32676.82
-0.00
38911.64 38911.42 38910.75 38909.65 38908.09 38906.09 38903.65
24
-0.01 0.03 -0.01 0.02
0.01 -0.01 -0.02 0.03 -0.02
19 0.02 -0.01 -0.01 -0.01 0.01 -0.03 -0.03
31121.34 31121.16 31120.62 31119.73 31118.53 31116.94 31114.96
-0.02 -0.01 0.00 -0.01 -0.01 -0.01 -0.02
37345.44 37345.20 37344.58 37343.52 37342.05 37340.14 37337-84
22
1,3,5Heptatriyne CH3(CXJzH (6) via (triethyl)silane was protective silyl group CH3(C=C)zH
-0.01 -0.01 -0.0 -0.00
-0.01 -0.01 -0.01
20 0.03 0.02 0.01 -0.00 0.04 0.03 -0.01
32677.34 32677.15 32676.59 32675.67 32674.41 32672.72 32670.67
0.02 -0.01 0.00 -0.00 0.01 0.00 0.03
38901.45 38901.21 38900.56 38899.45 38897.90 38895.94 38893.48
23
-0.00
-0.02 -0.02 -0.01 0.02 -0.00 -0.02
24
H(C=C)KN (1). Both these species, particularly CHs(C-C)XN, radio astronomy on the basis of laboratory data presented here. SYNTHESIS
-0.01 0.05 0.02 0.01 0.01 0.02 -0.01 0.03
2.4 pentadiyne
18
17
20 -0.01 -0.00 -0.05 -0.00 -0.02 0.01 0.01
23
1cyano J
(in MHz)
H&akfsne
18 -0.01 0.03 -0.02 0.01 -0.00 -0.00 0.05
21
34242.35 23242.20 34241.57 34240.61 34239.25 34237.48 34235.33 34232.79
I
State Line Frequencies
0.01 -0.01 -0.00 -0.01 -0.02 0.01 -0.03
may be detectable
by
OF POLYACETYLENES
was prepared from the readily available 1,3-pentadiyne, the alkyne chain extension technique (7) in which bromoethynylcoupled with the diyne in a Cadiot-Chodkiewicz reaction, the being removed subsequently with an aqueous base :
+ BrC=CSi(GHE)z
-
CH,(C-C)aSi(CzH&
Ho- CH3(C=C)gH. -----+
88
ALEXANDER
ET AL.
TABLE II Structural Data CH3(C-C)nCN
atom* AE(~=l8+17)~ B obc c C
7 a
'b
de Io
ref
27 371.92
'760.331
664.69866
3.9054 f 0.0003
27 754.63
770.962
655.53296
2.4620 f 0.0005
27 943.30
776.203
651.106'73 1.2449 f 0.0009
co
-
'd
27.932.39
775.900
651.36100
1.3440 l 0.0009
ce
27.699.50
769.431
656.83733
2.7069 f 0.0004
Nf
27.387.80
760.772
664.31336
3.8674 f 0.0003
a)
r
1.4534
1.456
1.2071
1.2081
2.5881p
1.3625 2.5846 1.2220
1.3829
1.3636
1.1605
1.1606
substituted atom
e) Normal specieaIoc649.56935
CH$-CEC-CEC-C-=N f 7 abcde
f) Error from 6r=O.O012/r
b) in MHz
g) =m
inamuA2
g
from 1,3 pentadiyne(13)
the rest from cyaoobutadiyoe (1)
c) Normal speciesBo*78.0401 d)
1‘
h) Only rM determined
Pentadiyne was also converted in high yield by the method of Jones and Lappert (8) to its triethylstannyl derivative which was then cyanodestannylated with cyanogen chloride : CH~(C~C)~H
+
(C2H6)$nN(C2H&
-
CH3(C=C)&r(C&)3
EXPERIMENTAL
$$
CH3(CzC)2CN.
DETAILS
The polyalkynes were stored at liquid nitrogen temperature and evaporated into the cell of the spectrometer taking care that the temperature of the samples never rose higher than necessary. Loss of sample tended to occur when these transfers were carried out. A Hewlett-Packard A8460 microwave spectrometer operating between 26.5 and 40 GHz was used. The cell was always cooled with dry ice and in general pressures of between 1 and 20 pm Hg were employed. Preparation
of
CHS(C=C)~H
A solution of (C2H6)$iC=CBr (23.0 g, 0.105 mole) in DMF (2.5 cm3) was added for 0.5 hr to a stirred mixture of EtNHz (7.5 g, 0.16 mole), NHzOHHCl (ca. 1.0 g), Cu~Clz (0.2 g), and CH,(C=C$H (7) (6.4 g, 0.1 mole) in DMF (40 cm3) maintained at 25’. After a further 0.5 hr the reaction mixture was shaken with ice-cold 2-M HCl and organic products were extracted with pentane (150 cm3). The pentane extracts were dried (MgSO,), concentrated under reduced pressure (80 mm Hg), and the oily residue was
SYMMETRIC
TOP POLYACETYLENES TABLE
89
III
First Excited State Transition Frequencies (MHz) J=25&24 Me(c*)fi k
4 1
v18 a)+ 38990.26
(hl
il)_ 38962.34
(i
oa
0-c
o-c
0.03
v17 36966.98
-0.07
'16 38982.94
0.04
38957.47
0.05
38954.93
0-c
o-c
"15 38964.76
0.04
0.01
38952.84
0.04 0.07
-0.00
36975.43
0.04
38962.91
-0.01
38968.20
0.03
38958.41
f2
*1
38976.73
0.03
38963.03
-0.06
38969.36
0.12
38958.76
0.02
fl
51
38975.13
0.02
38962.31
0.06
38967.69
-0.11
38958.00
0.16
38975.64
-0.01
38962.31
-0.11
38968.34
0.06
38958.00
0.05
f2
71
38974.17
0.02
38961.15
0.03
38966.81
0.02
38956.85
0.02
f4
N
38974.40
0.02
38961.23
-0.09
38966.99
*3
71
38972.72
0.03
38959.57
0.04
38965.34
f.5
fl
38972.72
-0.00
38959.69
-0.09
38965.34
f4
71
38970.71
-0.05
38957.48
-0.02
38963.37
f6
Ail
38970.71
0.07
38957.88
0.09
f5
71
38968.33
-0.04
38955.05
0.04
38955.28
-0.07
i.3 u
-0.07
0.05
38956.92
0.11
38955.33
-0.04
38955.33
0.07
0.04
38953.41
-0.06
38963.37
-0.02
38953.22
-0.06
38960.95
0.03
-0.09
17
il
38968.20
0.07
38960.88
-0.02
*6
Tl
36965.50
-0.03
38958.04
-0.01
36948.16
-0.21
*8
il
38965.23
0.05
38958.04
0.07
38948.16
0.11
f7
n
38954.93
0.20
38940.75
-0.34
f9
fl
*8
Tl
38958.50
-0.01
38951.09
0.12
Ltlo *1
38957.90
-0.05
38950.60
-0.19
71
38946.71
-0.05
38946.65
0.12
38941.78
-0.06
i9 ill
Ltl
ilO
+1
+12
il
chromatographed on alumina (La Porte, grade H) using pentane as eluent. Fractions rich in (CzH5)aSi(C=C)3CH3 (uv spectrum) were concentrated to leave a dark yellow oil (15 g; yield ca. 75yo) which was stored at -20” until required. Infrared (film) v (cm-‘): 2210 set, 2160 m, 2075 [-(C-C),-]; lH NMR (Ccl,) T: 8. OS (3H) [CHJ, 8.8-9.6 m (15H) [ (GH6)$i] : uv (hexane) h,,, (nm) (log e): 219.0 (4.03), 245.5 (2.78), 261 (2.25), 281 (2.34), 299 (2.32), 319 (2.06). Aqueous 2-M NaOH (10 cm3) was added with stirring to (C2Hb)$i(C--C)KH3 (10 g, 0.05 mole) dissolved in redistilled MeOH (200 cm3) held at 30” and desilylation was monitored by removing an aliquot at appropriate intervals and recording its uv spectrum after dilution with MeOH. When the band due to CH3(CX)aH at 268 nm was maximal, the reaction mixture was neutralized with dilute HCl and extracted with pentane. Concentration of the dried pentane extracts by careful evaporation under reduced pressure at -20” followed by chromatography yielded product-rich fractions
90
ALEXANDER TABLE Vibration-Rotation
ET AL. IV Parameters” CHS(C~C)PN
CQ(C=C)sH ”
v
Y
16
17
v
16
15
B0
778.2445l0.0005
178.2446i0.0007
776.S401 l 0.0008
770.040 +0.001
MHZ
%B
-1.2684t0.0006
-1.0236*0.0008
-1.3459l O.OOOS
-1.146 fO.OO1
MHZ
0.0092l 0.0005
0.0095*0.0007
0.0091l 0.0008
0.009 io.001
KHZ
4.442 iO.004
4.441* 0.006
4.406
l
0.013
4.41
M.02
KHZ
-0.013 f0.005
-0.031 l 0.009
-0.051
l
0.013
0.10
l 0.02
KHZ
6.59
l
0.02
6.08
io.03
KHZ
l4.(2
KHZ
@JO CL9 n P&O ap %J'o
0.0023*0.0008
6.14 io.01
/(q&1 568.0 6
l
2.0
0.9S36f0.0002
9.48 225.0
l
0.03
l
3.0
-0.005 M.001
561.0
0.9117*0.0007
i3.0
0.9607 *0.0005
235.0
KHZ
0.9688l0.0006
a) A0 was assumed to have the value 155.5GHEwhich was calculated from the srrwtureof CH&=Z)H.(13)
which were similarly concentrated at - 78’/10P3 mm Hg. The residue was flash distilled into a receiver maintained at - 196’ and the spectroscopically pure CH3(CXJ3H so obtained had the following characteristics; ir (film) v (cm-‘) 2200 s, 2230 s [-(C=C),-1, 3290 s (CX-H); ‘H NMR (Ccl,) T: 8.0 s (3H) [CH,], 8.12 s [CX-H] ; uv (pentane) X,,, (relative intensity): 254 (1.7), 268 (2.1), 284 (2.0), 302 (1.2). Preparation
of CH3 (C%)&N
(C2H5)2NSn(C2H6)3 (8) (5.1 g, 0.02 mole) was added for 10 min to CH3(C=C)zH (1.2 g, 0.02 mole) at 0”. The mixture was then heated and (CzHs)zNH was distilled out at atmospheric pressure to leave crude (CzHs)3Sn(C=C)EH3 (4.0 g) which was dissolved in CHzClz (10 cm3). This solution was added during 10 min to a stirred mixture of ClCN (2.0 g) and A1C13 (1.99 g) in CH&& (40 cm3) at 0”. The mixture was boiled under reflux for 15 min, then treated with ice-cold 2-M HCl (50 cm3). Organic products were extracted with ether and the extracts were dried (MgSOJ and concentrated in vacua. The oily residue was chromatographed on neutral alumina using pentane as eluent and product-rich fractions were concentrated again under reduced pressure, to leave CH~(C=C)ZCN (0.5 g, 37%) as a white solid, mp = 92”, which was stored at - 196’ until required. The product had the following spectral characteristics : ir (nujol) v cm-l: 2220 m, 2250 s [-(C=C),-1, 2240 s [-C=N]; lH NMR (Ccl,) T: 8.2 s (CH,). RESULTS
In Figs. la and lb the wide band scans of CH,(CSC)~H and CHI(C=C)ZCN are shown. There is very little difference between the B. values of these two species and thus their spectra are very similar. In Fig. 2a a medium resolution scan of the J = 25 + 24
SYMMETRIC
TOP
POLYACETYLENES
36.96
Hz I
1
36.96 I
1
36.94 I
I
CH~C~C-CSZ-C~C-H J=26+24
vi(1 = 1
and
“,T = 1
FIG. 3. The vibrational satellite structure can be assigned with the aid of a study of the way in which varying the Stark voltage affects the individual lines. Lines with low effective dipole moment (i.e., those with L = 0 and also the line for which f = 11 = fl) appear only at higher Stark voltage (b). The full assignment is given in Fig. 4.
transition of CHs(CK)aH with its intense vibrational satellite structure is shown. Structure as high as U, = 11 is clearly seen. The situation is very similar in the case of CHI(C-C)zCN as shown in Fig. 2b where the J = 23 + 22 transition is shown. One difference, however, can be seen in that the lines are considerably broader in the case of CH,(CzC)zCN. In running the spectra the power level was adjusted in order to eliminate saturation effects, and this broadening was still in evidence. The ground state structure for both molecules is typical of Csl, symmetric rotors and shows the expected simple k structure. The ground state frequencies are collected together in Table I for both CH,(C-C)aH and CH,(C=C)LYN. Although the spectra were quite dense it was just possible to detect 13C and ‘“N satellite patterns for the J = 18 t 17 transition of CH3(CK)2CN. This was found to
92
ALEXANDER
ET AL.
SYMMETRIC
TOP POLYACETYLENES
93
be the optimum transition, for although the higher frequency transitions are stronger they are more overlapped by vibrational satellites (Fig. lb) lending to difficulties in identification of the lines. These measurements are given in Table II. The structural data derived from these measurements are discussed in the following section. The vibrational satellites are quite complex in that several bending modes are of quite low frequency and contribute to the observed spectrum. However, it has been possible to analyze some of this structure, as discussed in the last section. STRUCTURE
DETERMINATION
Using the isotopic data for CH,(C=C),CN given in Table II it was possible to calculate substitution coordinates for all the heavy atoms except that of the central carbon atom and these results are also given in Table II together with data on H(CXJaCN. The central carbon atom is very close to the center of mass and thus its lines lie directly under those of the normal species. Thus we have only been able to determine the separation between Cb-Cd which is 2.5889 A, in close agreement with 2.5845 A, obtained for H(CEC)~CN. The structural results are consistent with those observed for H(GC)zCN (1). The spectrum of CH,(C=C)JH is much weaker and its isotopic satellite spectra were not detectable. VIBRATION-ROTATION
ANALYSIS
As can be seen in Figs. 1 and 2 the vibrational satellite patterns for these molecules are very rich and complicated. This is because there are several low frequency bending vibrations which are excited to high values of the vibrational quantum number, even at temperatures as low as - 7O’C. It was, however, possible to analyze the singly excited satellites for the two lowest frequency vibrations for each molecule. These are ~‘18= 1 and 2~17= 1 for CHB(C-C)~H, and 1~1~= 1 and zl15= 1 for CHs(C=C)XN. Only the frequencies of the _7 = 2.5 t 24 transition are given in Table III together with the assignments, although in all approximately 150 lines for each vibrational state (with J from 17 to 24 and K from 0 to ca. 6) have been measured and used to derive the vibrationrotation parameters (9). These parameters, calculated using the relations and procedure given by Careless and Kroto (10, 11), are given in Table IV. In assigning the lines it was very useful to note the rates at which various lines are modulated as a function of applied voltage. In Fig. 3 is shown a typical example of how the I-doublet lines for which h = It = fl and the line with i! = lt =f 1 (i.e., R = 0) are slow to appear as the Stark voltage is increased. In the tables of Ref. (10) I%= k - 1, where k is the quantum number associated with JA, the total angular momentum and 1, is the quantum number associated with the vibrational angular momentum (12). f can roughly be considered as a quantum number that gauges the rotational part (rather than the vibrational part) of this angular momentum about the symmetry axis. As a confirmation of the validity of the fitting procedure and the derived parameters, the J = 25 t 24 complex has been synthesized assuming Lorentzian line-shapes. The observed and synthesized spectra are shown in Fig. 4 for CH,(C=C)3H. Similar types of pattern are obtained for CHS(CX)2CN. The spectra shown relate directly to data given in Table III.
94
ALEXANDER
ET AL.
ACKNOWLEDGMENTS We thauk the Science Research Council for a Grant which assisted in the purchase of the spectrometer and for a maintenance award (to M.M.). RECEIVED: August
19, 1977 REFERENCES
1. A. J. ALEXANDER,H. W. KROTO, ANDD. R. M. WALTON, J. Mol. Spectrosc. 62, 175 (1976). 2. M. HUTCHINSON,H. W. KROTO, ANDD. R. M. WALTON, to appear. 3. L. W. AVERY, N. W. BROTEN, J. M. MALLEOD, T. OKA, AND H. W. KROTO, Astrophys. J. 205, L173 (1976). 4. C. KIRBY, H. W. KROTO, ANDD. R. M. WALTON, to appear. 5. H. W. KROTO, C. KIRBY, D. R. M. WALTON, L. W. AVERY, N. W. BROTEN, J. M. MACLEOD, AND T. OKA, Astrophys. J., in press. 6. J. B. ARMITAGE,C. L. COOKE,N. ENTWHISTLE,E. R. H. JONES,ANDM. C. WHITING,J. Chem. SOL, 1998 (1952). 7. R. EASTMONDAND D. R. M. WALTON, Tetrahedron 28, 4591 (1972). 8. K. JONESANDM. F. LAPPERT,J. Organometal. Chew 3,295 (1965). 9. M. MAIER, D. Phil. Thesis 1976, Univ. of Sussex. 10. A. J. CARELESSANDH. W. KROTO, J. Mol. Spectrosc. 57, 189 (1976). 11. A. J. CARELESSANDH. W. KKOTO,J. Mol. Spectrosc. 57, 198 (1975). 12. H. W. KROTO, “Molecular Rotation Spectra,” Wiley, New York/London, 1975. 13. C. C. COSTAIN,J. Chew Phys. 29, 864 (1958).
The Microwave 1,3,5-Heptatriyne,
Spectra
CH&=C)3H
of Symmetric
Top Polyacetylenes
and I-Cyano-2,4-pentadiyne,
CH,(C=C),CN
A. J. ALEXANDER, H. W. KROTO, M. MAIER, AND D. R. M. WALTON School of Moleculsr
Sciences,
University of Sussex, Brighton BNl
9QJ, United Kingdom
The rotational spectra of 1,3,%heptatriyne, CHa(CrC)aH and l-cyano-2,4-pentadiyne, CH)(C=C)ZCN have been studied in detail between 26.5 and 40.0 GHz. The molecules have
long linear chains of heavy atoms and show characteristic Ca, symmetric top spectra consisting of groups of R-branch lines at regular intervals separated by approximately 2Bo. Six isotopic modifications of CHr(C-C)&N have been detected in natural abundance allowing r, substitution structural data to be derived for this species. Long linear polyacetylenic chains are quite flexible and this dynamic property manifests itself in the appearance of extended sequences of complex vibrational satellites associated with the bending of the chain. The vibrational ground state spectra as well as several low frequency vibrational satellites have been analyzed yielding various vibration-rotation parameters. For CHs(CrC)3H BO = 778.2445 f 0.0007 and for CH,(CsC)XN Bo = 778.040 f 0.001 MHz. INTRODUCTION
In a previous study (1) an accurate analysis of the microwave spectrum of the long linear polyalkyne (polyacetylene) cyanobutadiyne, H(C=C)KN, was carried out. This molecule was of interest because it was the longest linear molecule thus far synthesized and showed interesting structural and dynamic properties associated with extended chains of atoms (2). Furthermore, on the basis of these laboratory measurements it has been possible to identify this species in the interstellar medium by radio astronomy (3). Since this work, we have synthesized the next member, cyanohexatriyne, H(CaC)3CN, (4) and also detected it in interstellar space (5). These species are the most complex interstellar species so far detected and are significant in that this complexity must be accounted for by any general theories of interstellar molecule formation. In this study we present the results of an extension of these techniques to the synthesis and study, by microwave spectroscopy, of two new CIV symmetric top polyalkynes: 1,3,5heptatriyne, CHI(CaC)BH, and 1-cyano-2,4-pentadiyne, CHI(C~C)GN. Microwave techniques are ideally suited to the study of such polyalkynes because under the low pressures required (- 10 pm Hg) the polymerization rate is slow and the molecules are stable for several hours. In the condensed phase at room temperature they polymerize rapidly. The long linear chains are quite flexible and the consequences of this in the spectra are of interest in that a large number of vibrational satellites occur, in fact, they domi84 OO22-2852/78/0701-0084$O2OO/O Copyright AU rights
@
1978
by Academic
of reproduction
in any
Press, form
Inc. reserved.
SYMMETRIC
TOP POLYACETYLENES
85 u^ “0 7
-21
1
d 0 2
86
ALEXANDER
ET AL.
nate the spectra. It has proven possible to analyze part of this structure and derive associated vibration-rotation parameters. Some of the isotopic satellites of CH,(CG)&N have been detected in natural abundance allowing substitution structure data to be derived. This has permitted the effects of chain lengthening on structure to be compared with similar effects in
:Hz
se:8
39.2
39.4
J=25c24 0
?HZ
I 36.4 b)
I
CHS-GC-CS-
I
I
36.2 C=N
J- 23t.22
I
I
I
36.0
I
35.3
1
FIG. 2. Medium resolution scans of the structure associated with a single I transition. CH,(CnC)aH (a) and CHa(CkC)KN (b). The lines of the latter are somewhat broader and considerably more intense. The bending vibrational satellites tend to lie in groups with the same value of II, and march out to high frequency as indicated. Same conditions as Fig. 1.
SYMMETRIC
TOP
POLYACETYLENES
TABLE Ground
1.395 k
J
17
28016.57 28016.45 28015.92 28015.15 28014.02 28012.58 28010.88
J 0
1 2 3 4 5 6 7 8 9
19 -0.02 0.09 0.03 -0.04 -0.04 0.01 0.01
31129.47 31129.30 31128.72 31127.88 31126.62 31125.05 31123.10
35798.79
-0.00
35798.62 35797.98 35796.95 35795.55 35793.71 35791.42 35788.81 35785.72 35782.20
0.03 0.00 -0.01 0.02 0.02 -0.03 0.02 -0.01 -0.06
37355.22 37355.00 37354.40 37353.30 37351.84 37349.91 37347.55 37344.77 37341.62 37337.95
29573.02 29572.96 29572.39 29571.47 29570.29 29568.62 29566.97
-0.01 0.03 -0.01 0.01 0.01 0.00 0.00 -0.00
22
0 1 2 3 4 5 6
28009.19 28009.04 28008.57 28007.82 28006.67 28005.30 28003.56 J
0
1 2 3 4 5 6
-0.04 -0.03 -0.03 0.01 -0.03 0.03 0.04
29565.29 29565.10 29564.60 29563.76 29562.61 29561.06 29559.22
0.03 0.03 0.01 0.01 0.00 0.02 0.01
35789.38 35789.20 35788.60 35787.58 35786.15 35784.33 35782.09
21
34233.41 34233.21 34232.62 34231.64 34230.28 34228.55 34226.41
32685.91 32685.78 32685.20 32684.25 32682.95 32681.28 32679.20 32676.82
-0.00
38911.64 38911.42 38910.75 38909.65 38908.09 38906.09 38903.65
24
-0.01 0.03 -0.01 0.02
0.01 -0.01 -0.02 0.03 -0.02
19 0.02 -0.01 -0.01 -0.01 0.01 -0.03 -0.03
31121.34 31121.16 31120.62 31119.73 31118.53 31116.94 31114.96
-0.02 -0.01 0.00 -0.01 -0.01 -0.01 -0.02
37345.44 37345.20 37344.58 37343.52 37342.05 37340.14 37337-84
22
1,3,5Heptatriyne CH3(CXJzH (6) via (triethyl)silane was protective silyl group CH3(C=C)zH
-0.01 -0.01 -0.0 -0.00
-0.01 -0.01 -0.01
20 0.03 0.02 0.01 -0.00 0.04 0.03 -0.01
32677.34 32677.15 32676.59 32675.67 32674.41 32672.72 32670.67
0.02 -0.01 0.00 -0.00 0.01 0.00 0.03
38901.45 38901.21 38900.56 38899.45 38897.90 38895.94 38893.48
23
-0.00
-0.02 -0.02 -0.01 0.02 -0.00 -0.02
24
H(C=C)KN (1). Both these species, particularly CHs(C-C)XN, radio astronomy on the basis of laboratory data presented here. SYNTHESIS
-0.01 0.05 0.02 0.01 0.01 0.02 -0.01 0.03
2.4 pentadiyne
18
17
20 -0.01 -0.00 -0.05 -0.00 -0.02 0.01 0.01
23
1cyano J
(in MHz)
H&akfsne
18 -0.01 0.03 -0.02 0.01 -0.00 -0.00 0.05
21
34242.35 23242.20 34241.57 34240.61 34239.25 34237.48 34235.33 34232.79
I
State Line Frequencies
0.01 -0.01 -0.00 -0.01 -0.02 0.01 -0.03
may be detectable
by
OF POLYACETYLENES
was prepared from the readily available 1,3-pentadiyne, the alkyne chain extension technique (7) in which bromoethynylcoupled with the diyne in a Cadiot-Chodkiewicz reaction, the being removed subsequently with an aqueous base :
+ BrC=CSi(GHE)z
-
CH,(C-C)aSi(CzH&
Ho- CH3(C=C)gH. -----+
88
ALEXANDER
ET AL.
TABLE II Structural Data CH3(C-C)nCN
atom* AE(~=l8+17)~ B obc c C
7 a
'b
de Io
ref
27 371.92
'760.331
664.69866
3.9054 f 0.0003
27 754.63
770.962
655.53296
2.4620 f 0.0005
27 943.30
776.203
651.106'73 1.2449 f 0.0009
co
-
'd
27.932.39
775.900
651.36100
1.3440 l 0.0009
ce
27.699.50
769.431
656.83733
2.7069 f 0.0004
Nf
27.387.80
760.772
664.31336
3.8674 f 0.0003
a)
r
1.4534
1.456
1.2071
1.2081
2.5881p
1.3625 2.5846 1.2220
1.3829
1.3636
1.1605
1.1606
substituted atom
e) Normal specieaIoc649.56935
CH$-CEC-CEC-C-=N f 7 abcde
f) Error from 6r=O.O012/r
b) in MHz
g) =m
inamuA2
g
from 1,3 pentadiyne(13)
the rest from cyaoobutadiyoe (1)
c) Normal speciesBo*78.0401 d)
1‘
h) Only rM determined
Pentadiyne was also converted in high yield by the method of Jones and Lappert (8) to its triethylstannyl derivative which was then cyanodestannylated with cyanogen chloride : CH~(C~C)~H
+
(C2H6)$nN(C2H&
-
CH3(C=C)&r(C&)3
EXPERIMENTAL
$$
CH3(CzC)2CN.
DETAILS
The polyalkynes were stored at liquid nitrogen temperature and evaporated into the cell of the spectrometer taking care that the temperature of the samples never rose higher than necessary. Loss of sample tended to occur when these transfers were carried out. A Hewlett-Packard A8460 microwave spectrometer operating between 26.5 and 40 GHz was used. The cell was always cooled with dry ice and in general pressures of between 1 and 20 pm Hg were employed. Preparation
of
CHS(C=C)~H
A solution of (C2H6)$iC=CBr (23.0 g, 0.105 mole) in DMF (2.5 cm3) was added for 0.5 hr to a stirred mixture of EtNHz (7.5 g, 0.16 mole), NHzOHHCl (ca. 1.0 g), Cu~Clz (0.2 g), and CH,(C=C$H (7) (6.4 g, 0.1 mole) in DMF (40 cm3) maintained at 25’. After a further 0.5 hr the reaction mixture was shaken with ice-cold 2-M HCl and organic products were extracted with pentane (150 cm3). The pentane extracts were dried (MgSO,), concentrated under reduced pressure (80 mm Hg), and the oily residue was
SYMMETRIC
TOP POLYACETYLENES TABLE
89
III
First Excited State Transition Frequencies (MHz) J=25&24 Me(c*)fi k
4 1
v18 a)+ 38990.26
(hl
il)_ 38962.34
(i
oa
0-c
o-c
0.03
v17 36966.98
-0.07
'16 38982.94
0.04
38957.47
0.05
38954.93
0-c
o-c
"15 38964.76
0.04
0.01
38952.84
0.04 0.07
-0.00
36975.43
0.04
38962.91
-0.01
38968.20
0.03
38958.41
f2
*1
38976.73
0.03
38963.03
-0.06
38969.36
0.12
38958.76
0.02
fl
51
38975.13
0.02
38962.31
0.06
38967.69
-0.11
38958.00
0.16
38975.64
-0.01
38962.31
-0.11
38968.34
0.06
38958.00
0.05
f2
71
38974.17
0.02
38961.15
0.03
38966.81
0.02
38956.85
0.02
f4
N
38974.40
0.02
38961.23
-0.09
38966.99
*3
71
38972.72
0.03
38959.57
0.04
38965.34
f.5
fl
38972.72
-0.00
38959.69
-0.09
38965.34
f4
71
38970.71
-0.05
38957.48
-0.02
38963.37
f6
Ail
38970.71
0.07
38957.88
0.09
f5
71
38968.33
-0.04
38955.05
0.04
38955.28
-0.07
i.3 u
-0.07
0.05
38956.92
0.11
38955.33
-0.04
38955.33
0.07
0.04
38953.41
-0.06
38963.37
-0.02
38953.22
-0.06
38960.95
0.03
-0.09
17
il
38968.20
0.07
38960.88
-0.02
*6
Tl
36965.50
-0.03
38958.04
-0.01
36948.16
-0.21
*8
il
38965.23
0.05
38958.04
0.07
38948.16
0.11
f7
n
38954.93
0.20
38940.75
-0.34
f9
fl
*8
Tl
38958.50
-0.01
38951.09
0.12
Ltlo *1
38957.90
-0.05
38950.60
-0.19
71
38946.71
-0.05
38946.65
0.12
38941.78
-0.06
i9 ill
Ltl
ilO
+1
+12
il
chromatographed on alumina (La Porte, grade H) using pentane as eluent. Fractions rich in (CzH5)aSi(C=C)3CH3 (uv spectrum) were concentrated to leave a dark yellow oil (15 g; yield ca. 75yo) which was stored at -20” until required. Infrared (film) v (cm-‘): 2210 set, 2160 m, 2075 [-(C-C),-]; lH NMR (Ccl,) T: 8. OS (3H) [CHJ, 8.8-9.6 m (15H) [ (GH6)$i] : uv (hexane) h,,, (nm) (log e): 219.0 (4.03), 245.5 (2.78), 261 (2.25), 281 (2.34), 299 (2.32), 319 (2.06). Aqueous 2-M NaOH (10 cm3) was added with stirring to (C2Hb)$i(C--C)KH3 (10 g, 0.05 mole) dissolved in redistilled MeOH (200 cm3) held at 30” and desilylation was monitored by removing an aliquot at appropriate intervals and recording its uv spectrum after dilution with MeOH. When the band due to CH3(CX)aH at 268 nm was maximal, the reaction mixture was neutralized with dilute HCl and extracted with pentane. Concentration of the dried pentane extracts by careful evaporation under reduced pressure at -20” followed by chromatography yielded product-rich fractions
90
ALEXANDER TABLE Vibration-Rotation
ET AL. IV Parameters” CHS(C~C)PN
CQ(C=C)sH ”
v
Y
16
17
v
16
15
B0
778.2445l0.0005
178.2446i0.0007
776.S401 l 0.0008
770.040 +0.001
MHZ
%B
-1.2684t0.0006
-1.0236*0.0008
-1.3459l O.OOOS
-1.146 fO.OO1
MHZ
0.0092l 0.0005
0.0095*0.0007
0.0091l 0.0008
0.009 io.001
KHZ
4.442 iO.004
4.441* 0.006
4.406
l
0.013
4.41
M.02
KHZ
-0.013 f0.005
-0.031 l 0.009
-0.051
l
0.013
0.10
l 0.02
KHZ
6.59
l
0.02
6.08
io.03
KHZ
l4.(2
KHZ
@JO CL9 n P&O ap %J'o
0.0023*0.0008
6.14 io.01
/(q&1 568.0 6
l
2.0
0.9S36f0.0002
9.48 225.0
l
0.03
l
3.0
-0.005 M.001
561.0
0.9117*0.0007
i3.0
0.9607 *0.0005
235.0
KHZ
0.9688l0.0006
a) A0 was assumed to have the value 155.5GHEwhich was calculated from the srrwtureof CH&=Z)H.(13)
which were similarly concentrated at - 78’/10P3 mm Hg. The residue was flash distilled into a receiver maintained at - 196’ and the spectroscopically pure CH3(CXJ3H so obtained had the following characteristics; ir (film) v (cm-‘) 2200 s, 2230 s [-(C=C),-1, 3290 s (CX-H); ‘H NMR (Ccl,) T: 8.0 s (3H) [CH,], 8.12 s [CX-H] ; uv (pentane) X,,, (relative intensity): 254 (1.7), 268 (2.1), 284 (2.0), 302 (1.2). Preparation
of CH3 (C%)&N
(C2H5)2NSn(C2H6)3 (8) (5.1 g, 0.02 mole) was added for 10 min to CH3(C=C)zH (1.2 g, 0.02 mole) at 0”. The mixture was then heated and (CzHs)zNH was distilled out at atmospheric pressure to leave crude (CzHs)3Sn(C=C)EH3 (4.0 g) which was dissolved in CHzClz (10 cm3). This solution was added during 10 min to a stirred mixture of ClCN (2.0 g) and A1C13 (1.99 g) in CH&& (40 cm3) at 0”. The mixture was boiled under reflux for 15 min, then treated with ice-cold 2-M HCl (50 cm3). Organic products were extracted with ether and the extracts were dried (MgSOJ and concentrated in vacua. The oily residue was chromatographed on neutral alumina using pentane as eluent and product-rich fractions were concentrated again under reduced pressure, to leave CH~(C=C)ZCN (0.5 g, 37%) as a white solid, mp = 92”, which was stored at - 196’ until required. The product had the following spectral characteristics : ir (nujol) v cm-l: 2220 m, 2250 s [-(C=C),-1, 2240 s [-C=N]; lH NMR (Ccl,) T: 8.2 s (CH,). RESULTS
In Figs. la and lb the wide band scans of CH,(CSC)~H and CHI(C=C)ZCN are shown. There is very little difference between the B. values of these two species and thus their spectra are very similar. In Fig. 2a a medium resolution scan of the J = 25 + 24
SYMMETRIC
TOP
POLYACETYLENES
36.96
Hz I
1
36.96 I
1
36.94 I
I
CH~C~C-CSZ-C~C-H J=26+24
vi(1 = 1
and
“,T = 1
FIG. 3. The vibrational satellite structure can be assigned with the aid of a study of the way in which varying the Stark voltage affects the individual lines. Lines with low effective dipole moment (i.e., those with L = 0 and also the line for which f = 11 = fl) appear only at higher Stark voltage (b). The full assignment is given in Fig. 4.
transition of CHs(CK)aH with its intense vibrational satellite structure is shown. Structure as high as U, = 11 is clearly seen. The situation is very similar in the case of CHI(C-C)zCN as shown in Fig. 2b where the J = 23 + 22 transition is shown. One difference, however, can be seen in that the lines are considerably broader in the case of CH,(CzC)zCN. In running the spectra the power level was adjusted in order to eliminate saturation effects, and this broadening was still in evidence. The ground state structure for both molecules is typical of Csl, symmetric rotors and shows the expected simple k structure. The ground state frequencies are collected together in Table I for both CH,(C-C)aH and CH,(C=C)LYN. Although the spectra were quite dense it was just possible to detect 13C and ‘“N satellite patterns for the J = 18 t 17 transition of CH3(CK)2CN. This was found to
92
ALEXANDER
ET AL.
SYMMETRIC
TOP POLYACETYLENES
93
be the optimum transition, for although the higher frequency transitions are stronger they are more overlapped by vibrational satellites (Fig. lb) lending to difficulties in identification of the lines. These measurements are given in Table II. The structural data derived from these measurements are discussed in the following section. The vibrational satellites are quite complex in that several bending modes are of quite low frequency and contribute to the observed spectrum. However, it has been possible to analyze some of this structure, as discussed in the last section. STRUCTURE
DETERMINATION
Using the isotopic data for CH,(C=C),CN given in Table II it was possible to calculate substitution coordinates for all the heavy atoms except that of the central carbon atom and these results are also given in Table II together with data on H(CXJaCN. The central carbon atom is very close to the center of mass and thus its lines lie directly under those of the normal species. Thus we have only been able to determine the separation between Cb-Cd which is 2.5889 A, in close agreement with 2.5845 A, obtained for H(CEC)~CN. The structural results are consistent with those observed for H(GC)zCN (1). The spectrum of CH,(C=C)JH is much weaker and its isotopic satellite spectra were not detectable. VIBRATION-ROTATION
ANALYSIS
As can be seen in Figs. 1 and 2 the vibrational satellite patterns for these molecules are very rich and complicated. This is because there are several low frequency bending vibrations which are excited to high values of the vibrational quantum number, even at temperatures as low as - 7O’C. It was, however, possible to analyze the singly excited satellites for the two lowest frequency vibrations for each molecule. These are ~‘18= 1 and 2~17= 1 for CHB(C-C)~H, and 1~1~= 1 and zl15= 1 for CHs(C=C)XN. Only the frequencies of the _7 = 2.5 t 24 transition are given in Table III together with the assignments, although in all approximately 150 lines for each vibrational state (with J from 17 to 24 and K from 0 to ca. 6) have been measured and used to derive the vibrationrotation parameters (9). These parameters, calculated using the relations and procedure given by Careless and Kroto (10, 11), are given in Table IV. In assigning the lines it was very useful to note the rates at which various lines are modulated as a function of applied voltage. In Fig. 3 is shown a typical example of how the I-doublet lines for which h = It = fl and the line with i! = lt =f 1 (i.e., R = 0) are slow to appear as the Stark voltage is increased. In the tables of Ref. (10) I%= k - 1, where k is the quantum number associated with JA, the total angular momentum and 1, is the quantum number associated with the vibrational angular momentum (12). f can roughly be considered as a quantum number that gauges the rotational part (rather than the vibrational part) of this angular momentum about the symmetry axis. As a confirmation of the validity of the fitting procedure and the derived parameters, the J = 25 t 24 complex has been synthesized assuming Lorentzian line-shapes. The observed and synthesized spectra are shown in Fig. 4 for CH,(C=C)3H. Similar types of pattern are obtained for CHS(CX)2CN. The spectra shown relate directly to data given in Table III.
94
ALEXANDER
ET AL.
ACKNOWLEDGMENTS We thauk the Science Research Council for a Grant which assisted in the purchase of the spectrometer and for a maintenance award (to M.M.). RECEIVED: August
19, 1977 REFERENCES
1. A. J. ALEXANDER,H. W. KROTO, ANDD. R. M. WALTON, J. Mol. Spectrosc. 62, 175 (1976). 2. M. HUTCHINSON,H. W. KROTO, ANDD. R. M. WALTON, to appear. 3. L. W. AVERY, N. W. BROTEN, J. M. MALLEOD, T. OKA, AND H. W. KROTO, Astrophys. J. 205, L173 (1976). 4. C. KIRBY, H. W. KROTO, ANDD. R. M. WALTON, to appear. 5. H. W. KROTO, C. KIRBY, D. R. M. WALTON, L. W. AVERY, N. W. BROTEN, J. M. MACLEOD, AND T. OKA, Astrophys. J., in press. 6. J. B. ARMITAGE,C. L. COOKE,N. ENTWHISTLE,E. R. H. JONES,ANDM. C. WHITING,J. Chem. SOL, 1998 (1952). 7. R. EASTMONDAND D. R. M. WALTON, Tetrahedron 28, 4591 (1972). 8. K. JONESANDM. F. LAPPERT,J. Organometal. Chew 3,295 (1965). 9. M. MAIER, D. Phil. Thesis 1976, Univ. of Sussex. 10. A. J. CARELESSANDH. W. KROTO, J. Mol. Spectrosc. 57, 189 (1976). 11. A. J. CARELESSANDH. W. KKOTO,J. Mol. Spectrosc. 57, 198 (1975). 12. H. W. KROTO, “Molecular Rotation Spectra,” Wiley, New York/London, 1975. 13. C. C. COSTAIN,J. Chew Phys. 29, 864 (1958).