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Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine
UftravioIet absorption vapor spectra of pyrazine and chloropyrazine R. C. HIRT Research Division, American Cyanamid Company, Stamford, Connecticut,U.S.A. (Received 30 September 1957)
Ab&&--The ultraviolet absorption spectra of pyrazine vapor and of chloropyrazine vapor are analyzed in terms of ground-state and excited-state vibrational frequencies and ~ombi~tious thereof. The “sharp” band systems of both pyrazine and chloropyrazine (attributed to “n-to-pi” transitions involving the nonbonding electrons of the heterocyclic nitrogen atoms) are analyzed. Only the “diffuse” system attributed to a “pi-to-pi” (benzenoid) transition of pyrazine vapor showed enough detail to permit any attempt at assignments. Extremely weak bands have been observed at lower energies in solution or liquid phases which show a substitutional “blue-shift” between pyrazine and chloropyrazine. These are attributed to a “n-to-pi” single~tsiplet transition. Infrared and Raman data for chloropyrazine are reported for comparison with the ultraviolet band spacings.
Introduction near-ultraviolet solution spectra of the diazines, their halogen and alkyl halide derivatives, and their behavior in various solvents have been described, along with a valence-bond treatment of the diazine rr-electrons [ 11. The two major band systems, called the “sharp” and the “diffuse” systems from their appearance in the vapor phase, were assigned to a nonbonding nitrogen electron transition and to a n-electron transition, respectively. The position of the zero-zero (O-O) band of the “sharp” system in the vapor phase and the approximate spacings of the principal upper-state vibrational bands have been reported for pyrazine (p-diazine) and for pyridazine (o-diazine) [Z]. The vapor spectrum of p~imi~ne (na-diazine) has been reported by UBER [3]. In this report, the details and assignments of the “sharp” and “diffuse” band systems of pyrazine vapor and of chloro pyrazine vapor are described. Since the publicati.on of the “Letter to the Editor” [Z] concerning the vapor spectra of the diazines was published, a Ph.D. thesis [P] was presented by Miss BETTY JANE FAX concerning the infrared and ultraviolet spectra of the diazines and pyridine. A microfilm copy of this thesis has been examined, but no data or assignments are used from it. Recently LORD et al. ]5] and ITO et al. [6] published data on the Raman spectrum of pyrazine, and IT0 et aZ. [7] have published data on the ‘Wiarp” ultraviolet vapor spectrum only. TEE
Expe~ent~ The vapor spectrum of pyrazine and chloropyrazine were photographed with a medium Hilger quartz spectrograph having a dispersion of 163 kaysersjmm (em-1/mm) at 31,000 K (cm-l), and with a large IIilger Littrow quartz spectrograph having a dispersion of 60 K/mm at 31,000 K. A water-cooled Nester hydrogen lamp operating from a regulated d.c. power supply served as the source. B’used quartz cells of 10, 20, SO,and 100 mm light path length and Eastman Kodak plates of types II-O and III-O were used. The vapor pressure of pyrazine and chloropyrazi~e were efficiently high to allow the use of these cell lengths. The cells were stoppered hut not evacuated. 114
Ultraviolet absorptionvapor spectra of pyrazine and chloropyrezine The measurement of the bands was done on tracings of the plates prepared on a Leeds and Northrup microphotometer of the Knorr-Albers type. Wavelength calibration was effected by use of an H-3 mercury lamp and a standard Pfund iron arc. The wavelengths of the bands were corrected to vacuum and converted into wavenumbers (kaysers) by use of KAYSER'S tables. From the conditions of the experiments and the measurements, the positions of the bands are probably not accurate to better than 4 K. The samples used were identical to those of the solution work [l] and had been purified by sublimation. The vapor spectra were also measured with a Cary automatic recording spectrophotometer, Model 11, using identical samples and cells. The instrument was operated at its lowest scanning
-2.5 26ooO
27ooO
28ooo
29030
K
Fig. 1. Singlet-triplet absorption bands of: Pyrezinep;
Chloropyrazine- - - -.
speed of 1 A/see and with very narrow slits (0.005 to 0.02 mm). These spectra closely resembled the microphotometer tracings, and measurement of the band positions yielded wavenumber differences in reasonably good agreement with the spectrographic data. The Gary was calibrated with a 4 W “Sterilamp” mercury lamp. The values reported in the Tables are almost entirely taken from the spectrographic data, although the Cary data were taken into consideration in making the band assignments. Solution spectra were obtained on both Cary and Beckman DU instruments, using cells of 50 and 100 mm and concentrated solutions. The infrared bands were obtained by Mr. JOHN WHALEN on a Perk&Elmer infrared spectrometer, Model 12C, using both NaCl and KBr optics. The solid pyrazine was melted, squeezed between two NaCl or KBr plates, and allowed to solidify before running. Chloropyrazine was run as a liquid between NaCl or KBr plates. The infrared band positions for pyrazine were in excellent agreement with those recently reported by LORD et al. [5] and so are not reproduced here, except where used in connection with the ultraviolet band assignments. The Raman date on chloropyrazine were obtained by Dr. R. F. STAMM,using a fast grating spectrograph [8].
Discussion (1) Observed electronic transitions of pyrazine Evidence set
of
3760 & position
three
for five electronic bands
has been
transitions observed
and chloropyrazine
of pyrazine in concentrated
has been obtained. cyclohexane
A weak
solutions
at
3670 and 3600 d (26,600, 27,250, and 27,800 K). The long wavelength and extreme weakness (E = O-015) indicate that these belong to a forChloropyrazine shows a shoulder at 3650 13.or singlet-triplet transition.
bidden 28,100 K with E = 0.05, thus displaying a substitutional “blue-shift.” These bands have been assigned to an n + rr*, singlet-triplet transition (Fig. 1) [9]. 115
R. C. HIRT The “sharp” band system, with its zero-zero vapor band at 30,879 K, has been nitrogen shown to arise from an n + 7~* transition involving the nonbonding electrons on the heterocycle nitrogen atoms [I]. The “diffuse” band system at shorter wavelength has similarly been explained as a n--t r* transition analogous to the familiar benzene 2600 A system. A fourth system indicates its presence by an upturn in the absorption curve near the short wavelength limit of the available instrumentation. KLEVENS and PLATT [lo] have published the vacuum ultraviolet spectrum of pyrazine in heptane solution, which shows the maximum of this band system to occur near 51,500 K (1940 A), and which indicates the presence of yet another band system above 58,000 K (below 1720 A). Four transitions for chloropyrazine are known. A shoulder attributed to an singlet-triplet transition was observed in the absorption spectrum of the n-r?, liquid. Like pyrazine, there is a “sharp” band system for the n + n* transition and a “diffuse” system of higher intensity for the first rr --t VT*(benzenoid) transition [I]. An up-turn of the solution absorption curves near 2150 A indicates the presence of a second r---f r * transition near 2000 A. No data in the vacuum ultraviolet are available. (2) Details of the grozlnd-state vibrations of pyrazine The “sharp” ultraviolet band system of pyrazine vapor shows enough detail to permit a vibrational analysis and comparison with the infrared and Raman spectra [5, 111. Pyrazine is a planar molecule of symmetry class V, (D&. Since it has a center of symmetry, the observed infrared and Raman bands should be The Z axis was chosen perpendicular to the plane of the ring, mutually exclusive. and the Y axis through the N atoms. (This choice of axes differs from those of LORD et al. and ITO et al. [5, 71.) The totally symmetric A,, modes would be expected to appear strongly as bands to the red of the zero-zero band, and they are found with spacings of 598, 1015, and 1228 K. Observed ultraviolet bands and assignments are given in Table 1. Fitting the corresponding benzene frequencies to the pyrazine bands is aided by the knowledge that the frequencies will not have undergone very large Other bands to the red of the zero-zero bands changes from benzene to pyrazine. have been assigned to ground-state frequencies, largely because of the “fit” of to modes their values; these are at 415, 852, 914, and 1414 K, corresponding 16b, lOa, 17a, and 19b. Ultraviolet bands to the “red” side of the O-O arise from vibrational states which are already excited at the time of the electronic transition, and the population of these states is regulated by the Boltzman factor. Thus at ordinary temperatures very few ultraviolet bands farther than 1000 K to the red of the zero-zero band are likely to be observed. (IT0 et al. [7] obtained bands as far as 2000 K with long-path cells at 140”.) (3) Details of the upper-state vibrations Unlike 1-O or l-l transitions, O-l transitions are not dependent upon the Boltzman factor for their probability. Transitions from the nonvibrating groundstate to vibrating upper-states are quite probable, and prominent bands are 116
Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine Table 1. “Sharp”
bands of the ultraviolet spectrum of pyrazine
-
Band position (K)
Difference from CL0 band
Estimated intensity *
Assignment
_
-
29,376 446 465 507 606 651 687 705 748 792 864 915 965 30,027 095 152 197 231 244 281 296 331 335 390 432 464 487 517 551 618 640 674 700 734 749 825 879 932 988 31,026 094 115 178 221 264 277 290
1503 1433 1414 1372? 1273 1228 1192 1174 1131 1087 1015 964 914 852 784 727 682 648 635 598 583 548? 544 489 447 415 392 362 328 261 239 205 179 145 130 54 0 53 109 147 215 236 299 342 385 398 411
-
vvw vvw vvw vvw vvw W W
0 -
1419 (19b)
0 0 -
1232 (9a) 2 x 598
0 0 0 0 0
1015 (1) 1015 + 53 914 (17a) 852 (lOa)? 415 - 362
VW VW VVW VW VW
m W
m
-
W W W W
m vw VW ma m
0 0 -
641 (6b) or (4) 598 (6a)
0 -
3 x 179
0 -
415 (16b)
0 0 0 -
362 (16a)f or 0 - 2 x 179? 805 + 474 (l-l of 11) 852 + 585 (l-l of 10a)
0 0 0 0 &O 0 0 -
415 + 236 (l-l of 16b) 362 + 215 (l-l of 16a) 362 + 236 1015 + 957 (l-l of 1)
W VW VW 8
m VS W
m VS
m m m vvs V8 W
415 + 474 362 + 474
W
0 + 215 (16a’) 0 + 236 (16b’)
m m m W
C’ of IT0 et al. [7]?$ 0 + 585 - 179?
m S m
117
R. C. HIRT Table 1 (continued) -_ Band position -
(K) 31,325 353 390 404 464 473 509 522 544 562 583 641 657 707 741 779 822 836 868 882 915 939 982 32,017 053 090 129 198 228 259 274 294 329 399 428 457 491 529 558 598 615 629 642 712 757 779 797
DiKerence from CO band
-
Estimated intensity*
Assignment
446 474 511 525 585 594 630 643 665 683 704 762 778 828 862 900 943 957 989 1003 1036 1060 1103 1138 1174 1211 1250 1319 1349 1380 1395 1418 1450 1520 1549 1678 1612 1650 1679 1719 1736 1750 1763 1833 1878 1900 1918
w El
0 + 236 + 215 or 0 + 630 - 179 0 -+ 474 (11’) or 0 + 2 x 236’f
W W
0 + 585 (6a’)
Vf3 W
m
0 + 630 (6b’)
W
m VW VW VW VW m m
0 + 665 (17a’) 0 + 215 + 474 0 + 236 + 474
W
m Ins m
0 0 0 0 0
+ + + + +
0
+
1138 - 2 x 179? 236 + 585 236 + 630 236 + 665 2 X 474 (OP c’ Of ITO)$ 957 (1’)
W
m m ms m
0 + 0 + 0 + 0 + 0 + 0 + (D’ 0 + 0 +
Vf3
m ms m W ms
m m W
ms W W W
474 + 585 630 + 474 1138 (9&x’) 2 x 585 630 + 585 1250 (14’) of ITO)$ 1138 + 236 1380 (19b’)
0 0 0 0 0 0
+ 957 + 4747 + 1450 (8b’)? f 1520 (8a’) + 957 + 585 + 957 + 630 f 1380 + 236 or 0 +
0 0 0 0 0 0
+ + t_ + + f
VW
m W W
m W W
1380 + 474 1138 + 585 1520 + 215 3 x 585 1520 + 236? 1250 + 585
VW W
0 + 2 x 957
W
118
179
1138 + 474
Ultraviolet absorption vapor spectra of pyrazine and chlorop,yrazine
(K)
Difference from 0-O band
32,823 858 902 921 973 33,024 076 132 154 203 296 343 432 479 561 630 882
1944 1974 2023 2042 2094 2145 2197 2253 2275 2324 2417 2462 2553 2600 2682 2751 3003
Band position
Estimated intensity *
Assignment
0 + 0 + 0 f 0 + 0 + 0+
m W W W VW W
1250 + 630 1380 + 585? 1250 + 665 1450 + 58501-O + 1138 + 957 1520+630
1380 + 665
VW \W
0 + 2 x 1138 0 + 1250 + 957 0 f 2 x 630 + 2 x 585
W VW VW vvw? vvw? vvw? vvw?
0 + 2 x 138OP
vvw? VVW?
* Key for intensities: vvw = very very weak; VW = very weak; w = weak; m = medium; ms = medium strong; s = strong; vs = very strong; vvs = very very strong. t LORD et al. [5] used 10a = 763 K. $ LORD et al. estimated 16a as 340 K. tj These assignments of IT0 et al. [7] were inserted during preparation of the manuscript.
observed.
The bands at separations of 215, 236, 474, 585, 630, 665, 957, 1138, and 1520 K from the O-O band have been assigned as upper-state vibrational bands. These bands and corresponding ground-state frequencies are summarized in Table 2, along with calculated and observed difference frequencies (l-l bands). Doubtful values or assignments are marked with a question mark (?). Considerable difficulty was encountered in assigning the prominent band at 398 K above the O-O and at 544 K below. (ITO et al. have assigned these to a different, forbidden n --t r* transition.) The difference frequencies are used in making assignments of the less prominent bands wherever their values permit. Similarly difference frequencies which arise between different modes are used in the assignments, as well as additive combinations. One notable feature of the pyrazine vapor spectrum is the lack of long progressions of the upper-state bands. Only a few 2 x and 3 x assignments could be made, and these were to rather weak bands. This lack of long progressions had been noted earlier [2] and used in the identification of the “sharp” band system or nonbonding nitrogen electron, transition [ 11. with an n-+n* 1250,
1380
(5) The “diffuse”
band of pyrazine
vapor
Only a few maxima could be resolved from the “diffuse” band system of pyrazine vapor. These bands could not be measured with anywhere near the 119
R. C. HIRT
precision of the bands of the “sharp” system. These data are shown in Table 3, along with the corresponding bands of the “sharp” system and tentative assignments. Monochloropyrazine. The near ultraviolet solution spectrum of monochloropyrazine was reported [I] along with data on other diazines. The “sharp” series of bands at longest wavelengths were assigned to a nonbonding nitrogen-electron transition, and the stronger, “diffuse” bands at shorter wavelengths were assigned to a n-electron (benzenoid) transition. The ultraviolet vapor spectrum of 2-chloropyrazine has not been previously reported. Table 2. Comparison of ground- and R = i.r. = U.V. =
-
Vibration
Mode
1 6a 8a 9a 16a 17a 1oa 19a 6b 11 16b 14 19b
4,
A 1U A 1U B 28 B 1U Bll7
B 1U B 1U B 3u B 3u
-
frequencies of the sharp bands of pyrazine Raman infrared ultraviolet
-
Ground-state
Upper-state
(K)
(K)
1015 598 1584 1228 362 914 852 1491 641 805 415 1345 1414
Alcl Al, Al,
upper-state observed in observed in observed in
u.v., u.v., R u.v., u.v.~ u.v., u.v.* i.r. R i.r. u.v., i.r. u.v.,
R i.r.
Ground-upper difference
68 13 64 90 147 249 267? 111 11 331 179 95 29
957 586 1520 1138 215 665 5853 1380 630 474 236 1250 1380
R i.r., R
i.r. i.r.
Observed difference freq. (K)
-
54
145 2399 261 328 179 -
-
-I
* LORD et al. [5] used 10a = 753 K. t LORD et al. estimated 10a as 340 K.
The infrared and Raman spectral data are summarized in Table 4, along with values for ground-state frequencies obtained from bands to the red of the zero-zero band of the ultraviolet vapor spectrum. The bands of the ultraviolet vapor spectrum are tabulated in Table 5, along with their differences (from the O-O band) and the assignments where they have been made. The V, symmetry class of pyrazine is Ground-state vibrations of chloropyrazine. reduced to the lower symmetry class C, or C,, for chloropyrazine, which has but one plane of symmetry (that through the ring of the molecule). All modes become A’ or A”. Assignment of vibrational modes becomes difficult and largely empirical. The assumption is made that certain modes are changed but little by the presence of if a “fit” of infrared, Raman and ground-state ultrathe heavy chlorine atom; violet frequencies is found to correspond to one of the unsubstituted pyrazine 120
Lltraviolet absorption vapor spe&a Table
-_...1 2
3 G, 6 7 8 9 I.0 If 1.2 13 14 15 16 17 18
va POr speotrum of P’.yrrazino T,Corresponding Possible Difference differeswe in &s&gmntmt from O-O sharp bands - - _______ -L ~__~
3. “Diffuse” bands of the ~tr~violet
Band position
Band no.
of pyrazine and obloropyrazine
(IEO
37,792 37,855 38,086 38,173 38,303 38,475 313,718 28,763 38,833 38,961 39,274 39,359 39,448 39,626 39,725 39,832 40,021 40,250 40,698 40,868
Relative iXWii!&ty to O-o .__ 0.17 0.18 0.09
o-09 0.13 0.24 0.93 I.00 o-93 0.43 O-36 0.38 0.37 o-43 o-52 o-41 0.30 O-18 0.14 O-O?
964 914 -
971 908 700 590 460 288 45 0 70 198 511 596 686 863 962 1069 1258 1487 1935 2105
I
I
I
0 -
598 4477 261 54 0 -
609 (Sa)
(l-l of iO&) (1-l of 1) O-O (16a’)
216? 511 586 683 862 957 1060 1350 -
0 0 0 0 0 0
1944 2094 -
914. (17a)
-+- 585 (6a’) + 215 -/- 474 i_ 236 -+- 630 t_ 957 (1’) + 474 + 685 -j- 1250 (14’)
0 + 0 f
1250 + 630 1138 + 957
_I_
frequencies, the mode is assigned to the analogous pyrazine mode. .A check may be made to see if the analogous vibrational mode in benzene is one which may be pictured as the chlorine and attached carbon atom holding still while the rest of the ring vibrates (an admittedly crude picture). “Fits” of infrared, Raman and ultraviolet frequencies which do not earrespond to a pyrazine frequency therefore are taken to be modes which are seriously affected by substitution; these must often remain unassigned. In Table 6 a comparison is made of the modes and ground-state frequencies found in pyrazine with those found in chloropyrazine. Of these values, only three appeared in ultraviolet, infrared and Raman data, though nine appeared in two of the techniques. Some ass~nments have been made on but one source; these are necessarily questionable. Since the probabilities or intensities of I-O and 1-L type transitions depend upon the population of the vibration ground-state (determined by the Boltzman factor), ground-state frequencies higher than 1000 or 1200 K are of little or no use in assigning ultraviolet yapor spectra bands. Use has been made of only the groundstate frequencies of 336, 432, 614, 654, 793, 832, 920 and 1007 K, though a tentative assignment was made of a difference frequency using 1225 (Ba). The use of 654 is somewhat doubtful, for the nearest ultraviolet band was at 661 and quite weak; however, good fits were obtained on three difference bands using 654 value. The weak band at 1194 K to the red of the O-O is probably a ground-state frequency because of its occurrence in the infrared and Raman spectra, but no
R. C. HIRT Table 4. Infrared, Raman,
-
Infrared band W
Estimated intensity
and ultraviolet data on ground-state Raman line
Ultraviolet band to red of O-o
Estimated intensity
0-Q
vibrations of chloropyrazine Estimated intensity
-
-
-
434 452 478 617 -
w W
m W
742 760 -
S 8
828 845 -
W S -
926 954 -
W W -
1013 1052 1136 -
S
1179 1192 1198 1225 1260 1290 1322 1338 1357 1390 1422 1462 1480 1520 1561 3060
m m
S S
W W
190 311 365 437 -
m
480 616 654 704 743 760 799 -
W W W S W 8 W S
m S
-
W
-
S -
432 -
W
477 614 661 -
m W W W
m vw -
843 9119 927 951 9923 1009 1046 1135 1158 1176 1192? -
W 3 W W P S S
VW VW
m vvw -
793 832 -
vvw vvw _-
92OP -
vvw -
1007 -
vvw
1194 -
W
-
m W
-
W 3 -
W S
-
S
1287 -
W -
1385 -
VW -
1461? 1482 1516 1561 -
7
-
-
-
VW
m W
-
-
-
-
-
mode could be assigned to it. It is likely a mode which has been greatly changed by the chlorine substitution. Upper-state vibrations of chloropyrazine. By use of the ground-state frequencies, the more prominent bands to the short wavelength side of the O-O band, and the agreement with difference-band assignments, a set of but five reasonable upperstate frequencies were arrived at. Ground- and upper-state frequencies are compared in Tables 6 and 7. 122
Ultraviolet absorption vapor spectra of pyrazine and cMoropyrazine Table 5. Bands and assignments for 2-chloropyrazine
-
Band position (K) 30,694 874 941 31,061 100 148 236 275 407 454 551 591 636 661 732 780 828 856 897 906 915 927 951 966 32,017 026 059 068 134 142 165 182 191 217 252 265 304 312 324 335 342 381 395 420 429 444 455
Difference from 60 band (K)
1374 1194 1127 1007 968 920 832 793 661 614 517 477 432 407 336 288 240 212 171 162 153 141 117 102 51 42 9 0 66 74 97 114 123 149 184 197 236 244 256 267 274 313 327 352 361 376 387
Intensity (estimated)
1 1
vapor
Assignment
vvw O-
1194?
o-
1007 (1)
VVW
O-
v3w
oooo-
920 (17a) 832 (10a) 793 (11) 654? (6b) Raman 654 614 (6a) 490 - 1007 or 0 + 274
W
vvw VVW VW
vvw vvw
m VW VW VW VW m VW VW
o+ oO-
432 (16b) 244 - 654 or 0 + 599 336 (16a)
o+
274 -
o+
490 3 x 976 490 2 x 599
o+
- . 793 _ 1007?
614
W
m W W
o-
W
0-t
W
o+
mS
o-
m
o+
m
- 654 or 0 + 274 51 - 832 - 614 51 - 654 (l-l of 6b)
-- 432 (l-l of 16b)
m vvs
O-0
w
o+
490 -
432?
0-t
490 +
274 -
VW
o+
VW
0+
976 976 244 -
832 or 0 + 793 51?
m m W
654
m
W
o+
336 + 490
W 8
m m a VW W
o+
o+
244 599 599 274 976 490
o+ o+ o+
976 - 614 1170 - 793 490 - 2 x 51?
o+ o+
o+ o+
(16a’?) + 274 - 336 (16b’) - 654? + 274 -
614 (l-l
432 or 0 + 976 -
VW W W W
123
of 6a -f- 16b’?)
654?
R. C.
HIRT
Table 5 (continued) Band position (K)
Difference from O-O band (K)
32,466 492 558 597 639 667 680 706 716 729 751 771 780 792 811 845 864 879 909 934 976 33,008 021 044 092 161 238 351 412 476 564 643 791 945 34,032 089 181 333 507 585 903 35,131
398 424 490 529 571 599 613 638 648 661 683 703 712 724 743 777 796 811 841 866 908 940 953 976 1024 1093 1170 1283 1344 1408 1496 1575 1723 1877 1964 2021 2113 2265 2439 2517 2835 3063
Intensity (estimated)
Assignment
W W 8
0 + 490 + 274 0 + 490 (ll’?)
336
W W 8
m m m W
0 0 0 0 0 0
+ + + + + +
976 + 599 - 1007? 599 (6a’) 1170 + 274 - 832 976 - 336 2 x 490 - 336 or 0 + 599 f 490 - 432
1170 f
274 -
7931
m m ms W W
0 +
m ms m
0 + 2 x 599 -
W
m
1170 -
432 336
0 + 244 + 599 0 + 1170 + 490 -
793
0 +
832
W W
m ms ms m ms
1170 + 599 -
0 + 976 (1’) 0 + 599 + 490 0 + 1170 (9a’)
W W W W W W
0 0 0 0
+ 244 f 1170 + 1170 + 976 - 654 + 976 + 599 + 1170 + 2 x 274
W W W W VW VW
0 + 1170 + 599 + 490 0 + 2 x 976 + 490
VW VW
0 + 2 x 1170 + 490?
VW
Only one or two multiples of upper-state frequencies could be found. Difference frequencies between upper- and lower-state were found for about twenty out of a only two of these were l-l transition of the same possible forty-five combinations; 124
Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine Table 6. Comparison of modes and ground-state
-
-
Pyrazinc mode
Designation
1 2 6a 8a 9a 16a 17a 1Oa 12 19a 3 6b 8b 11 16b 4 14 18b 19b
Al, Al, Al, AI, Al, A,, A 1U B 2s B 2u B IU B 18 Bl,
B IS B 1U B 1U B 38 B 3u B 3u B 321
Chloropyrazine mode
frequencies for pyrazine and chloropyrazine Chloropyrazine freq.
Pyrazine freq.
1007 3060 614 1561 1225 336 920? 832 1049 1481 1135? 654? 1523 793 432 760 1338? 1158? 1422?
1015 3055 596 1584 1228 362 914 852 1027 1491 1117 641 1523 805 417 755 1345 1150 1419
A’ A’ A’ A’ A’ A’ A’ A” A’ A” A’ A’ A’ A” A” A” A’ A’ A’
Clfound in
all i.r. all i.r. and R i.r. U.V. U.V.
ir. and U.V. i.r. and R i.r. and R i.r. and R R and u.v. i.r. and R U.V. and R all i.r. and R i.r. R i.r.
-
-
Table 7. Comparison of ground- and upper-state frequencies for chloropyrazine Remarks
I
I 1 6a Da 11 16b
1007 614 1225 i.r. 793 432
976 599 1170 490 274
31 15 55 304 158
Too near 0-O to be resolved Too near &O to be resolved 51 162
~ mode. Persistent difference values appear to be lacking, although a few assignments of doubtful status could be made with the fifty-one differences. This is probably characteristic of a molecule of low symmetry. Acknowledgements-The assistance of Mr. J. WHALEN in obtaining the infrared spectra and of Mr. W. L. DUTTON in photographing certain of the ultraviolet vapor spectra is gratefully acknowledged. Dr. R. C. LORD very kindly permitted the examination and use of the Raman spectral data on pyrazine [l l] prior to the publication of these data, and Dr. R. F. STAMM very kindly furnished t,he Raman spectral data on chloropyrazine. The many discussions with Dr. F. HALVERSON were especially helpful, as were those with Dr. M. KASHA and Dr. L. GOODMAN; the singlet-triplet bands of pyrazine were originally observed and their assignment suggested by Dr. KASHA [9]. 2
125
R. C. HIRT: Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll]
HALVERSON F. and HIRT R. C. J. Chem. Phys. 1951 19 711. HALVERSON F. and HIRT R. C. J. Chem. Phys. 1949 17 1165. UBER F. M. J. Chem. Phys. 1941 9 777. FAX B. J. Near Ultraviolet Absorption Spectra of Pyridine, Pyridine-d,, and the Diazines under High Dispersion. Ph.D. Thesis, Massachusetts Institute of Technology, 1950. LORD R. C., MARSTON A. L. and MILLER F. A. Spectrochim. Acta 1957 9 113. ITO M., SHIMADA R., KURAISHI T. and MIZUSHIMA W. J. Chem. Phys. 1956 25 597. ITO M., SHIMADA R., KURAISHI T. and MIZUSHIMA W. J. Chem. Phys. 1957 26 1508. STAMM R. F. Inckstr. Engng. Chem. (Anal.) 1945 17 318. GOODMAN L. and KASHAM M. J. Mol. Spectroec. In Press. Survey of Vacuum UZtravioZet Spectra Tech. Rept. ONR KLEVENS H. B. and PLAY J. R. Contract N60RI-20, Task Order IX, Project NR 019 101, 1954. LORD R. C. Private communication Sept. 2, 1954.
126
Ab&&--The ultraviolet absorption spectra of pyrazine vapor and of chloropyrazine vapor are analyzed in terms of ground-state and excited-state vibrational frequencies and ~ombi~tious thereof. The “sharp” band systems of both pyrazine and chloropyrazine (attributed to “n-to-pi” transitions involving the nonbonding electrons of the heterocyclic nitrogen atoms) are analyzed. Only the “diffuse” system attributed to a “pi-to-pi” (benzenoid) transition of pyrazine vapor showed enough detail to permit any attempt at assignments. Extremely weak bands have been observed at lower energies in solution or liquid phases which show a substitutional “blue-shift” between pyrazine and chloropyrazine. These are attributed to a “n-to-pi” single~tsiplet transition. Infrared and Raman data for chloropyrazine are reported for comparison with the ultraviolet band spacings.
Introduction near-ultraviolet solution spectra of the diazines, their halogen and alkyl halide derivatives, and their behavior in various solvents have been described, along with a valence-bond treatment of the diazine rr-electrons [ 11. The two major band systems, called the “sharp” and the “diffuse” systems from their appearance in the vapor phase, were assigned to a nonbonding nitrogen electron transition and to a n-electron transition, respectively. The position of the zero-zero (O-O) band of the “sharp” system in the vapor phase and the approximate spacings of the principal upper-state vibrational bands have been reported for pyrazine (p-diazine) and for pyridazine (o-diazine) [Z]. The vapor spectrum of p~imi~ne (na-diazine) has been reported by UBER [3]. In this report, the details and assignments of the “sharp” and “diffuse” band systems of pyrazine vapor and of chloro pyrazine vapor are described. Since the publicati.on of the “Letter to the Editor” [Z] concerning the vapor spectra of the diazines was published, a Ph.D. thesis [P] was presented by Miss BETTY JANE FAX concerning the infrared and ultraviolet spectra of the diazines and pyridine. A microfilm copy of this thesis has been examined, but no data or assignments are used from it. Recently LORD et al. ]5] and ITO et al. [6] published data on the Raman spectrum of pyrazine, and IT0 et aZ. [7] have published data on the ‘Wiarp” ultraviolet vapor spectrum only. TEE
Expe~ent~ The vapor spectrum of pyrazine and chloropyrazine were photographed with a medium Hilger quartz spectrograph having a dispersion of 163 kaysersjmm (em-1/mm) at 31,000 K (cm-l), and with a large IIilger Littrow quartz spectrograph having a dispersion of 60 K/mm at 31,000 K. A water-cooled Nester hydrogen lamp operating from a regulated d.c. power supply served as the source. B’used quartz cells of 10, 20, SO,and 100 mm light path length and Eastman Kodak plates of types II-O and III-O were used. The vapor pressure of pyrazine and chloropyrazi~e were efficiently high to allow the use of these cell lengths. The cells were stoppered hut not evacuated. 114
Ultraviolet absorptionvapor spectra of pyrazine and chloropyrezine The measurement of the bands was done on tracings of the plates prepared on a Leeds and Northrup microphotometer of the Knorr-Albers type. Wavelength calibration was effected by use of an H-3 mercury lamp and a standard Pfund iron arc. The wavelengths of the bands were corrected to vacuum and converted into wavenumbers (kaysers) by use of KAYSER'S tables. From the conditions of the experiments and the measurements, the positions of the bands are probably not accurate to better than 4 K. The samples used were identical to those of the solution work [l] and had been purified by sublimation. The vapor spectra were also measured with a Cary automatic recording spectrophotometer, Model 11, using identical samples and cells. The instrument was operated at its lowest scanning
-2.5 26ooO
27ooO
28ooo
29030
K
Fig. 1. Singlet-triplet absorption bands of: Pyrezinep;
Chloropyrazine- - - -.
speed of 1 A/see and with very narrow slits (0.005 to 0.02 mm). These spectra closely resembled the microphotometer tracings, and measurement of the band positions yielded wavenumber differences in reasonably good agreement with the spectrographic data. The Gary was calibrated with a 4 W “Sterilamp” mercury lamp. The values reported in the Tables are almost entirely taken from the spectrographic data, although the Cary data were taken into consideration in making the band assignments. Solution spectra were obtained on both Cary and Beckman DU instruments, using cells of 50 and 100 mm and concentrated solutions. The infrared bands were obtained by Mr. JOHN WHALEN on a Perk&Elmer infrared spectrometer, Model 12C, using both NaCl and KBr optics. The solid pyrazine was melted, squeezed between two NaCl or KBr plates, and allowed to solidify before running. Chloropyrazine was run as a liquid between NaCl or KBr plates. The infrared band positions for pyrazine were in excellent agreement with those recently reported by LORD et al. [5] and so are not reproduced here, except where used in connection with the ultraviolet band assignments. The Raman date on chloropyrazine were obtained by Dr. R. F. STAMM,using a fast grating spectrograph [8].
Discussion (1) Observed electronic transitions of pyrazine Evidence set
of
3760 & position
three
for five electronic bands
has been
transitions observed
and chloropyrazine
of pyrazine in concentrated
has been obtained. cyclohexane
A weak
solutions
at
3670 and 3600 d (26,600, 27,250, and 27,800 K). The long wavelength and extreme weakness (E = O-015) indicate that these belong to a forChloropyrazine shows a shoulder at 3650 13.or singlet-triplet transition.
bidden 28,100 K with E = 0.05, thus displaying a substitutional “blue-shift.” These bands have been assigned to an n + rr*, singlet-triplet transition (Fig. 1) [9]. 115
R. C. HIRT The “sharp” band system, with its zero-zero vapor band at 30,879 K, has been nitrogen shown to arise from an n + 7~* transition involving the nonbonding electrons on the heterocycle nitrogen atoms [I]. The “diffuse” band system at shorter wavelength has similarly been explained as a n--t r* transition analogous to the familiar benzene 2600 A system. A fourth system indicates its presence by an upturn in the absorption curve near the short wavelength limit of the available instrumentation. KLEVENS and PLATT [lo] have published the vacuum ultraviolet spectrum of pyrazine in heptane solution, which shows the maximum of this band system to occur near 51,500 K (1940 A), and which indicates the presence of yet another band system above 58,000 K (below 1720 A). Four transitions for chloropyrazine are known. A shoulder attributed to an singlet-triplet transition was observed in the absorption spectrum of the n-r?, liquid. Like pyrazine, there is a “sharp” band system for the n + n* transition and a “diffuse” system of higher intensity for the first rr --t VT*(benzenoid) transition [I]. An up-turn of the solution absorption curves near 2150 A indicates the presence of a second r---f r * transition near 2000 A. No data in the vacuum ultraviolet are available. (2) Details of the grozlnd-state vibrations of pyrazine The “sharp” ultraviolet band system of pyrazine vapor shows enough detail to permit a vibrational analysis and comparison with the infrared and Raman spectra [5, 111. Pyrazine is a planar molecule of symmetry class V, (D&. Since it has a center of symmetry, the observed infrared and Raman bands should be The Z axis was chosen perpendicular to the plane of the ring, mutually exclusive. and the Y axis through the N atoms. (This choice of axes differs from those of LORD et al. and ITO et al. [5, 71.) The totally symmetric A,, modes would be expected to appear strongly as bands to the red of the zero-zero band, and they are found with spacings of 598, 1015, and 1228 K. Observed ultraviolet bands and assignments are given in Table 1. Fitting the corresponding benzene frequencies to the pyrazine bands is aided by the knowledge that the frequencies will not have undergone very large Other bands to the red of the zero-zero bands changes from benzene to pyrazine. have been assigned to ground-state frequencies, largely because of the “fit” of to modes their values; these are at 415, 852, 914, and 1414 K, corresponding 16b, lOa, 17a, and 19b. Ultraviolet bands to the “red” side of the O-O arise from vibrational states which are already excited at the time of the electronic transition, and the population of these states is regulated by the Boltzman factor. Thus at ordinary temperatures very few ultraviolet bands farther than 1000 K to the red of the zero-zero band are likely to be observed. (IT0 et al. [7] obtained bands as far as 2000 K with long-path cells at 140”.) (3) Details of the upper-state vibrations Unlike 1-O or l-l transitions, O-l transitions are not dependent upon the Boltzman factor for their probability. Transitions from the nonvibrating groundstate to vibrating upper-states are quite probable, and prominent bands are 116
Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine Table 1. “Sharp”
bands of the ultraviolet spectrum of pyrazine
-
Band position (K)
Difference from CL0 band
Estimated intensity *
Assignment
_
-
29,376 446 465 507 606 651 687 705 748 792 864 915 965 30,027 095 152 197 231 244 281 296 331 335 390 432 464 487 517 551 618 640 674 700 734 749 825 879 932 988 31,026 094 115 178 221 264 277 290
1503 1433 1414 1372? 1273 1228 1192 1174 1131 1087 1015 964 914 852 784 727 682 648 635 598 583 548? 544 489 447 415 392 362 328 261 239 205 179 145 130 54 0 53 109 147 215 236 299 342 385 398 411
-
vvw vvw vvw vvw vvw W W
0 -
1419 (19b)
0 0 -
1232 (9a) 2 x 598
0 0 0 0 0
1015 (1) 1015 + 53 914 (17a) 852 (lOa)? 415 - 362
VW VW VVW VW VW
m W
m
-
W W W W
m vw VW ma m
0 0 -
641 (6b) or (4) 598 (6a)
0 -
3 x 179
0 -
415 (16b)
0 0 0 -
362 (16a)f or 0 - 2 x 179? 805 + 474 (l-l of 11) 852 + 585 (l-l of 10a)
0 0 0 0 &O 0 0 -
415 + 236 (l-l of 16b) 362 + 215 (l-l of 16a) 362 + 236 1015 + 957 (l-l of 1)
W VW VW 8
m VS W
m VS
m m m vvs V8 W
415 + 474 362 + 474
W
0 + 215 (16a’) 0 + 236 (16b’)
m m m W
C’ of IT0 et al. [7]?$ 0 + 585 - 179?
m S m
117
R. C. HIRT Table 1 (continued) -_ Band position -
(K) 31,325 353 390 404 464 473 509 522 544 562 583 641 657 707 741 779 822 836 868 882 915 939 982 32,017 053 090 129 198 228 259 274 294 329 399 428 457 491 529 558 598 615 629 642 712 757 779 797
DiKerence from CO band
-
Estimated intensity*
Assignment
446 474 511 525 585 594 630 643 665 683 704 762 778 828 862 900 943 957 989 1003 1036 1060 1103 1138 1174 1211 1250 1319 1349 1380 1395 1418 1450 1520 1549 1678 1612 1650 1679 1719 1736 1750 1763 1833 1878 1900 1918
w El
0 + 236 + 215 or 0 + 630 - 179 0 -+ 474 (11’) or 0 + 2 x 236’f
W W
0 + 585 (6a’)
Vf3 W
m
0 + 630 (6b’)
W
m VW VW VW VW m m
0 + 665 (17a’) 0 + 215 + 474 0 + 236 + 474
W
m Ins m
0 0 0 0 0
+ + + + +
0
+
1138 - 2 x 179? 236 + 585 236 + 630 236 + 665 2 X 474 (OP c’ Of ITO)$ 957 (1’)
W
m m ms m
0 + 0 + 0 + 0 + 0 + 0 + (D’ 0 + 0 +
Vf3
m ms m W ms
m m W
ms W W W
474 + 585 630 + 474 1138 (9&x’) 2 x 585 630 + 585 1250 (14’) of ITO)$ 1138 + 236 1380 (19b’)
0 0 0 0 0 0
+ 957 + 4747 + 1450 (8b’)? f 1520 (8a’) + 957 + 585 + 957 + 630 f 1380 + 236 or 0 +
0 0 0 0 0 0
+ + t_ + + f
VW
m W W
m W W
1380 + 474 1138 + 585 1520 + 215 3 x 585 1520 + 236? 1250 + 585
VW W
0 + 2 x 957
W
118
179
1138 + 474
Ultraviolet absorption vapor spectra of pyrazine and chlorop,yrazine
(K)
Difference from 0-O band
32,823 858 902 921 973 33,024 076 132 154 203 296 343 432 479 561 630 882
1944 1974 2023 2042 2094 2145 2197 2253 2275 2324 2417 2462 2553 2600 2682 2751 3003
Band position
Estimated intensity *
Assignment
0 + 0 + 0 f 0 + 0 + 0+
m W W W VW W
1250 + 630 1380 + 585? 1250 + 665 1450 + 58501-O + 1138 + 957 1520+630
1380 + 665
VW \W
0 + 2 x 1138 0 + 1250 + 957 0 f 2 x 630 + 2 x 585
W VW VW vvw? vvw? vvw? vvw?
0 + 2 x 138OP
vvw? VVW?
* Key for intensities: vvw = very very weak; VW = very weak; w = weak; m = medium; ms = medium strong; s = strong; vs = very strong; vvs = very very strong. t LORD et al. [5] used 10a = 763 K. $ LORD et al. estimated 16a as 340 K. tj These assignments of IT0 et al. [7] were inserted during preparation of the manuscript.
observed.
The bands at separations of 215, 236, 474, 585, 630, 665, 957, 1138, and 1520 K from the O-O band have been assigned as upper-state vibrational bands. These bands and corresponding ground-state frequencies are summarized in Table 2, along with calculated and observed difference frequencies (l-l bands). Doubtful values or assignments are marked with a question mark (?). Considerable difficulty was encountered in assigning the prominent band at 398 K above the O-O and at 544 K below. (ITO et al. have assigned these to a different, forbidden n --t r* transition.) The difference frequencies are used in making assignments of the less prominent bands wherever their values permit. Similarly difference frequencies which arise between different modes are used in the assignments, as well as additive combinations. One notable feature of the pyrazine vapor spectrum is the lack of long progressions of the upper-state bands. Only a few 2 x and 3 x assignments could be made, and these were to rather weak bands. This lack of long progressions had been noted earlier [2] and used in the identification of the “sharp” band system or nonbonding nitrogen electron, transition [ 11. with an n-+n* 1250,
1380
(5) The “diffuse”
band of pyrazine
vapor
Only a few maxima could be resolved from the “diffuse” band system of pyrazine vapor. These bands could not be measured with anywhere near the 119
R. C. HIRT
precision of the bands of the “sharp” system. These data are shown in Table 3, along with the corresponding bands of the “sharp” system and tentative assignments. Monochloropyrazine. The near ultraviolet solution spectrum of monochloropyrazine was reported [I] along with data on other diazines. The “sharp” series of bands at longest wavelengths were assigned to a nonbonding nitrogen-electron transition, and the stronger, “diffuse” bands at shorter wavelengths were assigned to a n-electron (benzenoid) transition. The ultraviolet vapor spectrum of 2-chloropyrazine has not been previously reported. Table 2. Comparison of ground- and R = i.r. = U.V. =
-
Vibration
Mode
1 6a 8a 9a 16a 17a 1oa 19a 6b 11 16b 14 19b
4,
A 1U A 1U B 28 B 1U Bll7
B 1U B 1U B 3u B 3u
-
frequencies of the sharp bands of pyrazine Raman infrared ultraviolet
-
Ground-state
Upper-state
(K)
(K)
1015 598 1584 1228 362 914 852 1491 641 805 415 1345 1414
Alcl Al, Al,
upper-state observed in observed in observed in
u.v., u.v., R u.v., u.v.~ u.v., u.v.* i.r. R i.r. u.v., i.r. u.v.,
R i.r.
Ground-upper difference
68 13 64 90 147 249 267? 111 11 331 179 95 29
957 586 1520 1138 215 665 5853 1380 630 474 236 1250 1380
R i.r., R
i.r. i.r.
Observed difference freq. (K)
-
54
145 2399 261 328 179 -
-
-I
* LORD et al. [5] used 10a = 753 K. t LORD et al. estimated 10a as 340 K.
The infrared and Raman spectral data are summarized in Table 4, along with values for ground-state frequencies obtained from bands to the red of the zero-zero band of the ultraviolet vapor spectrum. The bands of the ultraviolet vapor spectrum are tabulated in Table 5, along with their differences (from the O-O band) and the assignments where they have been made. The V, symmetry class of pyrazine is Ground-state vibrations of chloropyrazine. reduced to the lower symmetry class C, or C,, for chloropyrazine, which has but one plane of symmetry (that through the ring of the molecule). All modes become A’ or A”. Assignment of vibrational modes becomes difficult and largely empirical. The assumption is made that certain modes are changed but little by the presence of if a “fit” of infrared, Raman and ground-state ultrathe heavy chlorine atom; violet frequencies is found to correspond to one of the unsubstituted pyrazine 120
Lltraviolet absorption vapor spe&a Table
-_...1 2
3 G, 6 7 8 9 I.0 If 1.2 13 14 15 16 17 18
va POr speotrum of P’.yrrazino T,Corresponding Possible Difference differeswe in &s&gmntmt from O-O sharp bands - - _______ -L ~__~
3. “Diffuse” bands of the ~tr~violet
Band position
Band no.
of pyrazine and obloropyrazine
(IEO
37,792 37,855 38,086 38,173 38,303 38,475 313,718 28,763 38,833 38,961 39,274 39,359 39,448 39,626 39,725 39,832 40,021 40,250 40,698 40,868
Relative iXWii!&ty to O-o .__ 0.17 0.18 0.09
o-09 0.13 0.24 0.93 I.00 o-93 0.43 O-36 0.38 0.37 o-43 o-52 o-41 0.30 O-18 0.14 O-O?
964 914 -
971 908 700 590 460 288 45 0 70 198 511 596 686 863 962 1069 1258 1487 1935 2105
I
I
I
0 -
598 4477 261 54 0 -
609 (Sa)
(l-l of iO&) (1-l of 1) O-O (16a’)
216? 511 586 683 862 957 1060 1350 -
0 0 0 0 0 0
1944 2094 -
914. (17a)
-+- 585 (6a’) + 215 -/- 474 i_ 236 -+- 630 t_ 957 (1’) + 474 + 685 -j- 1250 (14’)
0 + 0 f
1250 + 630 1138 + 957
_I_
frequencies, the mode is assigned to the analogous pyrazine mode. .A check may be made to see if the analogous vibrational mode in benzene is one which may be pictured as the chlorine and attached carbon atom holding still while the rest of the ring vibrates (an admittedly crude picture). “Fits” of infrared, Raman and ultraviolet frequencies which do not earrespond to a pyrazine frequency therefore are taken to be modes which are seriously affected by substitution; these must often remain unassigned. In Table 6 a comparison is made of the modes and ground-state frequencies found in pyrazine with those found in chloropyrazine. Of these values, only three appeared in ultraviolet, infrared and Raman data, though nine appeared in two of the techniques. Some ass~nments have been made on but one source; these are necessarily questionable. Since the probabilities or intensities of I-O and 1-L type transitions depend upon the population of the vibration ground-state (determined by the Boltzman factor), ground-state frequencies higher than 1000 or 1200 K are of little or no use in assigning ultraviolet yapor spectra bands. Use has been made of only the groundstate frequencies of 336, 432, 614, 654, 793, 832, 920 and 1007 K, though a tentative assignment was made of a difference frequency using 1225 (Ba). The use of 654 is somewhat doubtful, for the nearest ultraviolet band was at 661 and quite weak; however, good fits were obtained on three difference bands using 654 value. The weak band at 1194 K to the red of the O-O is probably a ground-state frequency because of its occurrence in the infrared and Raman spectra, but no
R. C. HIRT Table 4. Infrared, Raman,
-
Infrared band W
Estimated intensity
and ultraviolet data on ground-state Raman line
Ultraviolet band to red of O-o
Estimated intensity
0-Q
vibrations of chloropyrazine Estimated intensity
-
-
-
434 452 478 617 -
w W
m W
742 760 -
S 8
828 845 -
W S -
926 954 -
W W -
1013 1052 1136 -
S
1179 1192 1198 1225 1260 1290 1322 1338 1357 1390 1422 1462 1480 1520 1561 3060
m m
S S
W W
190 311 365 437 -
m
480 616 654 704 743 760 799 -
W W W S W 8 W S
m S
-
W
-
S -
432 -
W
477 614 661 -
m W W W
m vw -
843 9119 927 951 9923 1009 1046 1135 1158 1176 1192? -
W 3 W W P S S
VW VW
m vvw -
793 832 -
vvw vvw _-
92OP -
vvw -
1007 -
vvw
1194 -
W
-
m W
-
W 3 -
W S
-
S
1287 -
W -
1385 -
VW -
1461? 1482 1516 1561 -
7
-
-
-
VW
m W
-
-
-
-
-
mode could be assigned to it. It is likely a mode which has been greatly changed by the chlorine substitution. Upper-state vibrations of chloropyrazine. By use of the ground-state frequencies, the more prominent bands to the short wavelength side of the O-O band, and the agreement with difference-band assignments, a set of but five reasonable upperstate frequencies were arrived at. Ground- and upper-state frequencies are compared in Tables 6 and 7. 122
Ultraviolet absorption vapor spectra of pyrazine and cMoropyrazine Table 5. Bands and assignments for 2-chloropyrazine
-
Band position (K) 30,694 874 941 31,061 100 148 236 275 407 454 551 591 636 661 732 780 828 856 897 906 915 927 951 966 32,017 026 059 068 134 142 165 182 191 217 252 265 304 312 324 335 342 381 395 420 429 444 455
Difference from 60 band (K)
1374 1194 1127 1007 968 920 832 793 661 614 517 477 432 407 336 288 240 212 171 162 153 141 117 102 51 42 9 0 66 74 97 114 123 149 184 197 236 244 256 267 274 313 327 352 361 376 387
Intensity (estimated)
1 1
vapor
Assignment
vvw O-
1194?
o-
1007 (1)
VVW
O-
v3w
oooo-
920 (17a) 832 (10a) 793 (11) 654? (6b) Raman 654 614 (6a) 490 - 1007 or 0 + 274
W
vvw VVW VW
vvw vvw
m VW VW VW VW m VW VW
o+ oO-
432 (16b) 244 - 654 or 0 + 599 336 (16a)
o+
274 -
o+
490 3 x 976 490 2 x 599
o+
- . 793 _ 1007?
614
W
m W W
o-
W
0-t
W
o+
mS
o-
m
o+
m
- 654 or 0 + 274 51 - 832 - 614 51 - 654 (l-l of 6b)
-- 432 (l-l of 16b)
m vvs
O-0
w
o+
490 -
432?
0-t
490 +
274 -
VW
o+
VW
0+
976 976 244 -
832 or 0 + 793 51?
m m W
654
m
W
o+
336 + 490
W 8
m m a VW W
o+
o+
244 599 599 274 976 490
o+ o+ o+
976 - 614 1170 - 793 490 - 2 x 51?
o+ o+
o+ o+
(16a’?) + 274 - 336 (16b’) - 654? + 274 -
614 (l-l
432 or 0 + 976 -
VW W W W
123
of 6a -f- 16b’?)
654?
R. C.
HIRT
Table 5 (continued) Band position (K)
Difference from O-O band (K)
32,466 492 558 597 639 667 680 706 716 729 751 771 780 792 811 845 864 879 909 934 976 33,008 021 044 092 161 238 351 412 476 564 643 791 945 34,032 089 181 333 507 585 903 35,131
398 424 490 529 571 599 613 638 648 661 683 703 712 724 743 777 796 811 841 866 908 940 953 976 1024 1093 1170 1283 1344 1408 1496 1575 1723 1877 1964 2021 2113 2265 2439 2517 2835 3063
Intensity (estimated)
Assignment
W W 8
0 + 490 + 274 0 + 490 (ll’?)
336
W W 8
m m m W
0 0 0 0 0 0
+ + + + + +
976 + 599 - 1007? 599 (6a’) 1170 + 274 - 832 976 - 336 2 x 490 - 336 or 0 + 599 f 490 - 432
1170 f
274 -
7931
m m ms W W
0 +
m ms m
0 + 2 x 599 -
W
m
1170 -
432 336
0 + 244 + 599 0 + 1170 + 490 -
793
0 +
832
W W
m ms ms m ms
1170 + 599 -
0 + 976 (1’) 0 + 599 + 490 0 + 1170 (9a’)
W W W W W W
0 0 0 0
+ 244 f 1170 + 1170 + 976 - 654 + 976 + 599 + 1170 + 2 x 274
W W W W VW VW
0 + 1170 + 599 + 490 0 + 2 x 976 + 490
VW VW
0 + 2 x 1170 + 490?
VW
Only one or two multiples of upper-state frequencies could be found. Difference frequencies between upper- and lower-state were found for about twenty out of a only two of these were l-l transition of the same possible forty-five combinations; 124
Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine Table 6. Comparison of modes and ground-state
-
-
Pyrazinc mode
Designation
1 2 6a 8a 9a 16a 17a 1Oa 12 19a 3 6b 8b 11 16b 4 14 18b 19b
Al, Al, Al, AI, Al, A,, A 1U B 2s B 2u B IU B 18 Bl,
B IS B 1U B 1U B 38 B 3u B 3u B 321
Chloropyrazine mode
frequencies for pyrazine and chloropyrazine Chloropyrazine freq.
Pyrazine freq.
1007 3060 614 1561 1225 336 920? 832 1049 1481 1135? 654? 1523 793 432 760 1338? 1158? 1422?
1015 3055 596 1584 1228 362 914 852 1027 1491 1117 641 1523 805 417 755 1345 1150 1419
A’ A’ A’ A’ A’ A’ A’ A” A’ A” A’ A’ A’ A” A” A” A’ A’ A’
Clfound in
all i.r. all i.r. and R i.r. U.V. U.V.
ir. and U.V. i.r. and R i.r. and R i.r. and R R and u.v. i.r. and R U.V. and R all i.r. and R i.r. R i.r.
-
-
Table 7. Comparison of ground- and upper-state frequencies for chloropyrazine Remarks
I
I 1 6a Da 11 16b
1007 614 1225 i.r. 793 432
976 599 1170 490 274
31 15 55 304 158
Too near 0-O to be resolved Too near &O to be resolved 51 162
~ mode. Persistent difference values appear to be lacking, although a few assignments of doubtful status could be made with the fifty-one differences. This is probably characteristic of a molecule of low symmetry. Acknowledgements-The assistance of Mr. J. WHALEN in obtaining the infrared spectra and of Mr. W. L. DUTTON in photographing certain of the ultraviolet vapor spectra is gratefully acknowledged. Dr. R. C. LORD very kindly permitted the examination and use of the Raman spectral data on pyrazine [l l] prior to the publication of these data, and Dr. R. F. STAMM very kindly furnished t,he Raman spectral data on chloropyrazine. The many discussions with Dr. F. HALVERSON were especially helpful, as were those with Dr. M. KASHA and Dr. L. GOODMAN; the singlet-triplet bands of pyrazine were originally observed and their assignment suggested by Dr. KASHA [9]. 2
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R. C. HIRT: Ultraviolet absorption vapor spectra of pyrazine and chloropyrazine
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll]
HALVERSON F. and HIRT R. C. J. Chem. Phys. 1951 19 711. HALVERSON F. and HIRT R. C. J. Chem. Phys. 1949 17 1165. UBER F. M. J. Chem. Phys. 1941 9 777. FAX B. J. Near Ultraviolet Absorption Spectra of Pyridine, Pyridine-d,, and the Diazines under High Dispersion. Ph.D. Thesis, Massachusetts Institute of Technology, 1950. LORD R. C., MARSTON A. L. and MILLER F. A. Spectrochim. Acta 1957 9 113. ITO M., SHIMADA R., KURAISHI T. and MIZUSHIMA W. J. Chem. Phys. 1956 25 597. ITO M., SHIMADA R., KURAISHI T. and MIZUSHIMA W. J. Chem. Phys. 1957 26 1508. STAMM R. F. Inckstr. Engng. Chem. (Anal.) 1945 17 318. GOODMAN L. and KASHAM M. J. Mol. Spectroec. In Press. Survey of Vacuum UZtravioZet Spectra Tech. Rept. ONR KLEVENS H. B. and PLAY J. R. Contract N60RI-20, Task Order IX, Project NR 019 101, 1954. LORD R. C. Private communication Sept. 2, 1954.
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