Absorption Spectrum of Pyridine-D

Jaya V. Shukla, K. N. Upadhya, and S. N. Thakur Department of Spectroscopy, Banaras Hindu University, Varanasi-5, India

(Received 14 May 1971; revision received 13 September 1971)

The absorption spectrum of v-~* system of pyridine-D5 molecule has been measured in the range 33790-36990 cm -~, on the Hilger large quartz spectrograph. A large number of sharp bands have been measured and analyzed in terms of ground and excited state frequencies which have been compared with the corresponding frequencies of pyridine-H5 molecule. INDEX HEADING: Visible, ultraviolet spectroscopy; Absorption spectra of pyridine-D5

INTRODUCTION

The fundamental vibrational frequencies of pyri- dine-H5 molecule have been studied by various workers, the assignments in the ir and Raman spectra have been completed by Corresen et al. 1 and Long and George, ~ who have made the assignments for the pyridine-D5 molecule also. The latter workers have also calculated these frequencies theoretically and have drawn the different modes of vibration for the -D5 molecule. The monodeuteropyridines have been studied by Anderson et al. 3 In the ~r-~'* system of pyridine-Hs, corresponding to the ~B:u - IA l g system of benzene, part of the vibronic structure has been studied in the absorption spectrum by Henri and Angenot, 4 Spouer, 5 and Sponer and Stucklem 6 The latter workers have observed a large number of vibronic bands and analyzed them in terms of the vibrational fundamentals. A satisfactory analysis of the vibronic structure under low resolution, however, is possible if data from one or more of the isotopic molecules are available. The present study of the electronic absorption spectrum for the structure of pyridine-D5 was under taken with a view of fulfil this requirement.

I. EXPERIMENTAL METHOD

The pyridine-D5 liquid used for the present studies was BDH make (deuteration 99.999%). Absorption cell lengths of 20, 75, and 100 cm were used with the sample container at temperatures ranging from 15 to 25°C, for different regions of the spectrum. The spec- trum was recorded on a Hilger large quartz prism spectrograph (El) through a 30-~ slitwidth, on Kodak B-10 plates. A Beckman hydrogen lamp was used as the source of the continuum. The bands in the region 2830-2960 A were best developed with cell lengths of 75 cm, with the container bulb at room temperature. The bands in the region 2780-2940 A were best developed with the cell length of 20 cm, with the bulb at ice temperature. The bands in the region 2690- 2920 A were best developed with cell length 20 cm, and the bulb temperature at -15°C. Each spectrum was recorded in 4 h., and was centrally superimposed by an iron-arc spectrum for providing standard lines for measurements.

A. Study of Plates

The pyridine-D5 absorption spectrum (Fig. 1) con- sists of slightly redegraded bands in the region 33 790- 36 990 cm -1 and looks very similar to the corresponding pyridine-H5 spectrum. The bands may be classified into two major groups, one having sharp, almost line- like bands and the other having diffused bands. A diffused intensity maxima at a separation of 8 cm -~ to the lower frequency side is associated with a number of bands. A much weaker maximum is associated with some of the bands at a separation of about 15 cm -1 to the higher-frequency side. Nearly all the intense vibronic bands show sequences of difference fre- quencies, most common difference frequencies being 19, 30, 40, and 36 cm -~.

II. RESULTS AND DISCUSSION

The pyridine molecule (H5 or D6) belongs to the C2, point group, with the C2 axis (Z axis) passing through the nitrogen and the parapositioned C atoms. The 27 fundamental vibrations are divided into (10al+9b2) planar and (Sbl+3a2) nonplanar modes. It is assumed that the molecular symmetry remains unchanged in the upper electronic state. The present absorption system is attributed to a 1B2-1A1 electronic transition (allowed). 7's The electronic spectrum of pyridine is expected to resemble that of benzene, its • - electrons retaining the memory of benzene's D6h symmetry. The present electronic transition being an allowed one, a strong 0 - 0 band is expected, with strong vibronic bands involving totally symmetrical fundamentals. The vibronic assignments have been made on the basis of information obtained from the Raman and ir spectra of this molecule, and confirm to the similar band system for pyridine-Hs. 6 The effect of deuteration on the magnitudes of the vibrational frequencies has been utilized in assigning them to the various fundamental modes. The detailed analysis is given in Table I and some of the important features are described below.

A. The 0 - 0 Band

The 0-O band has been chosen as the intense band at 34 951 cm -1, appearing at the lowest bulb tempera-

Volume 26, Number 2, 1972 APPLIED SPECTROSCOPY 283

0-1010 O- 891

0-822

0-622

O- 580

0-:570

0-529

0-8 0-0(54951) 0-15 0-160 0-209

0-512 0-560

0-671

0-819 O- 852 0 -955 0-2x512 0-2x560

0-5x512

0-5x560

0-2x953

0-4x512

Fro. 1. The ~r-~r* absorption system of pyridine --D5 spectrum, recorded on an E, large quartz spectrograph.

tures, showing a red shift of 182 cm -~ from the 0 - 0 band of pyridine-H5 molecule. At higher bulb tempera- tures and pressures, it appears as a broad diffused band tha t resolves at lower pressures and bulb tem- peratures into a very sharp, almost linelike band, with a very small redegradation, and strong but less sharp (slightly diffused) bands at separations of - 8 and + 1 5 cm -1 from the main peak. This character is not very common in the other bands, and it is not possible to explain them as difference frequencies. A similar feature has been observed for pyridine-H5 also 6 but with only one diffused peak at a separation of + 10 cm- ' from the main peak. This band has been assigned at + 1 0 cm -~ to the K structure of the pyridine-H5 molecule. However, it is difficult to suggest the origin of these additional peaks, with the resolution and dis- persion used in the present case, but we feel tha t these may be ascribed to rotat ional s t ructure of the bands in question, as done by Sponer 6 for pyridine-Hs.

B. Totally Symmetrical Fundamentals

A majori ty of the intense vibronic bands may be explained in terms of the total ly symmetrical vibra-

tional frequencies. All of these, in the excited state, have an appearance like tha t of the 0 - 0 band.

1. The Ring-Breathing Mode

This mode has been identified as having the excited state frequency of 935 cm -~, and has been correlated to the 962 cm -~ ground-state frequency, showing a decrease of about 3% on electronic excitation, which is in fair agreement with the observations made for several subst i tuted benzene molecules (p-dibromo- benzenes - H 4 and --D4, p-fluoroaniline, p-fluoro- phenol, etc.). These assignments are well supported by the change in their values upon deuterat ion (Table II). The bands involving the ring-breathing mode were found to be relatively weak in the present case, as was observed for pyridine-Hs.

2. Planar Ring-Bending Modes

The 582-cm -~ ground-state and the 512-cm -~ excited-state frequencies are involved in the strongest bands of the spectrum. The 512-cm -~ frequency ap- pears up to four quanta, all the overtones appearing with a diffused peak at a separation of - 2 cm -~ from the main band. These frequencies have been assigned to a symmetric planarring-bending mode, correlated with the pyridine-H5 frequencies of 601 and 542 cm- ' in the ground and the excited states, respectively. The frequency change in the two states on deuterat ion is the same, both the molecules showing a lowering of about 7% in this frequency on electronic excitation, this fact supporting this correlation. The difference frequency of about 68 cm -~ may be explained as due to this mode ( 0 - 5 8 2 + 5 1 2 ) . Another very strong excited-state frequency of 953 cm -~, appearing up to two quanta, has been assigned to the C-C-C triangular bending mode (1010 cm -1) in the ground state. Both the excited state quanta show a s t ructure similar to the 0 - 0 band. This frequency is involved in a large number of vibronic bands, and it may explain the difference frequency of about 56 cm -~ by a combina- tion 0-1010+953. The corresponding pyridine-H5 frequencies are 1031 and 995 cm -~ in the ground and the excited states, respectively.

3. Planar C-H Bending Modes

Of the two modes observed of this type, one is assigned to the 891-cm -1 ground-state frequency cor- related to the 952-cm -1 excited-state frequency. A lowering of about 27.5% is bbserved in these fre- quencies due to deuterat ion (Table II), in both the states, which is expected for nuclei involving large motions of the H atom and has been observed for other subst i tuted benzenes. The other C - H planar mode (al type) is assigned to the 822-cm -1 ground-state fre- quency, 1,2 showing a frequency drop of about 23% on deuteration. I t is correlated to the 819-cm -1 excited- state frequency.

284 Volume 26, Number 2, 1972

Table I. Ultraviolet absorption spectrum of pyridine-D.~.

Differ- Fre - enee

quen- f rom cies O - O

Rela t ive • i n t ens i t y (in b a n d 15°C 20°C - 1 0 ° C cm-1) (cm -i) A s s i g n m e n t s

1 33 792 1 33 849 3 33 940 2 34 000

1 1 34 045

3 34 059 2 34 095 3 1 34 123 2 34 149 2 34 192 3 34 219 2 34 253 3 34 275

3 1 1 34 315 2 34 328 4 34 349 9 2.5 2 34 370 2 34 387 2 34 406 4 1 34 436 4 34 491 2.5 34 478 7.5 2 1 34 492 2 34 508

1 34 525 1 34 526

2.5 1 34 531 3 1 1 34 568 3.5 1 34 580 5 1.5 34 621 3 34 628 8.5 3.5 1 34 639

2 34 651 4 2 34 672 2 1 34 689 2 1.5 34 701 2.5 1.5 34 729 2.5 1 34 744 2 34 765" 8.5 4 2 34 801 3 34 804 5 2 34 820 5 1.5 34 824

0.5 34 865 5.5 2.5 34 882 6 4 2 34 894 4 2 34 906

4 34 920

4.5 2.5 34 931 5.5 34 942 5.5 10 8 34 950 2 3 2 34 965

2 34 975 2 34 985 2 3.5 2 34 986 2 2 1 35 000 2.5 4 2 35 012 1.5 1.5 35 O3O 1.5 2 1 35 054 5 8 6 35 070 3 3 1.5 35 081

4 1.5 35 088 1.5 1.4 35 107

2 2 35 114

- -1158 0 - 2 X 5 8 0 --1101 0 - 2 X 5 6 0 --1010 0-1010 - - 950 0 -891 -56 ; 0 + 2 0 9 -

2)<580 -- 9 0 5 0 - 5 8 0 - 2 X 1 5 9 ;

0 -329-580 - - 891 0 -891 ; 0 - 1 0 1 0 + 2 0 -- 855 0 - 5 9 7 ; 1 5 9 - - 822 0-822 -- 801 0 -622 -159 -15 - 758 0 -580 -159 -19 -- 731 0 -580-159 ; 0 -2 )<370 -- 697 0 - 6 9 7 ; 0 - 3 2 9 - 3 7 0 -- 675 0 - 6 9 7 + 1 5 ;

0 - 5 8 0 - 5 6 - 3 0 - 8 -- 635 0 -4 )<159 ; 0 - 6 2 2 - 8 -- 622 0-622 -- 601 0 - 2 ) < 5 8 0 + 5 6 0 -- 580 0-580 -- 563 0 - 5 8 0 + 1 5 -- 544 0 - 1 0 1 0 + 5 1 2 - 3 0 - 1 9 -- 514 0 - 1 0 1 0 + 5 1 2 - 1 9 -- 499 0 - 1 0 1 0 + 5 1 2 -- 472 0-3)<159 -- 458 0 - 5 8 0 + 1 2 0 -- 442 0 - 3 2 9 - 2 X 5 6 -- 425 0 - 3 2 9 - 2 ) < 5 6 + 1 5 - - 424 0 - 3 2 9 - 5 6 - 3 0 - 8 -- 419 0 - 3 2 9 - 5 6 - 3 0 -- 382 0 - 3 7 0 - 8 ; 0 - 8 9 1 + 5 1 2 -- 370 0-370; 0 - 5 8 0 + 2 0 9 -- 329 0-329 -- 322 0 - 2 ) < 1 5 9 - 8 -- 311 0 -2)<159;

0 - 8 2 2 + 5 1 2 - 299 0 - 2 > ( 1 5 9 + 3 0 - 1 9 -- 278 0 - 2 ) < 1 5 9 + 6 2 - 3 0 -- 261 0 - 8 2 2 + 5 6 0 -- 249 0 -4 )<56-30 -- 221 0 - 8 9 1 + 6 7 1 -- 206 0 - 3 2 9 + 1 2 0 -- 185 0 - 6 9 7 + 5 1 2 -- 159 0 -159 ; 0 + 2 0 9 - 3 7 0 -- 146 0 - 3 2 9 + 2 0 9 - 3 0 -- 130 0 - 3 2 9 + 2 0 9 - 8 -- 126 0 - 3 2 9 + 2 0 9 - - 85 0 - 3 2 9 + 2 ) < 1 5 0 -- 68 0 -68 ; 0 - 5 8 0 + 5 1 2 -- 56 0 - 5 6 ; 0 - 1 0 1 0 + 9 5 3 -- 44 0 + 1 2 0 - 1 5 9 ;

0 + 8 5 2 - 8 9 1 ; 0 + 2 X 5 6 0 - 2 X 5 8 0

- - 30 0 -30 ; 0 - 6 9 7 + 6 7 1 ; 0 - 1 9 - 8

-- 19 0 - 5 8 0 + 5 6 0 - 8 0 -8 - - 0 0 - 0

15 0 + 2 X 5 1 2 - 1 0 1 0 ; 0 + 1 5

25 0 + 120 -56 -30 -8 35 0 + 120-56-30 36 0 + 9 5 3 - 8 9 1 - 3 0 50 0 + 2 0 9 - 1 5 0 62 0 + 9 5 3 - 8 9 1 80 0 + 2 X 1 2 0 - 1 5 9

104 0 + 1 2 0 - 1 9 120 0 + 1 2 0 131 0 + 9 5 3 - 8 2 2 138 0 + 2 0 9 - 5 6 - 1 9 157 0 + 2 0 9 - 5 6 ;

0 + 8 5 2 - 6 7 9 164 0 + 2 0 9 - 3 0 - 1 9

Re la t ive • i n t e n s i t y 15°C 20°C - 1 0 ° C

Fre- quen-

cies (in

em -i)

Differ- ence f rom O - O b a n d

(era -1) A s s i g n m e n t s

2 2 35 127 1 1 35 134

2.5 3 35 152

2.5 3 2 35 159 1 1 1 3 5 167 ½ 2 35 183

½ 2 35 190 1 2 35 199 ½ 2 35 208

1 8 4 35 219 2 35 225 2.5 35 235

1 5 2 35 249 2 1 35 274

3 1 1 35 290

2 2.5 2 35 303

6 2 35 315 6 2 35 325 2 1 35 339 2.5 35 374 3 2 35 394 3 2 35 403 8 35 414 5 2 35 444 2 2 35 454 6 8 35 462 3 2 35 475

2 2 35 502 6 10 35 510 2 2 35 522

1 35 540 3 1.5 35 552 1.5 35 563 2 35 567 7 2.5 35 579 5 2 35 588 3 2 35 602 ½ 35 613 7 8 35 521

1 35 634 3 2 35 657 2.5 1 35 662

1 35 682 2 35 696 2 2.5 35 702 2 35 718 2 3 35 724 2.5 3.5 35 738 2.5 2.5 35 751

2.5 2 35 761 3 4 35 769 2 4 35 781 2 4 35 790 2.5 1 35 725 2.5 1 35 802 ½ 35 815

2 3 35 846 1.5 2.5 35 856 ½ 2 35 885 1 8 35 895 1 10 35 903

177 0 + 2 0 9 - 3 0 184 0 + 2 X 120-56;

0 + 5 1 2 - 3 2 9 202 0 + 2 0 9 - 8 ;

0 + 3 X 120-159 ; 0 + 2 X 522-822

209 0 + 2 0 9 217 0 + 8 5 2 - 5 8 0 - 5 6 233 0 + 2 X 120-8;

0 - 6 2 2 + 8 5 2 240 0 + 2 X 1 2 0 249 0 + 8 5 2 - 5 8 0 - 1 9 258 0 + 2 > ( 2 0 9 + 5 5 9 ;

0 + 9 5 3 - 6 9 7 269 0 + 8 5 2 - 5 8 0 275 0 + 9 5 3 - 2 X 3 2 9 - 1 9 285 0 + 9 5 3 - 2 X 3 2 9 - 3 299 0 + 9 5 3 - 2 X 3 2 9 324 0 + 3 X 120-30;

0-697 + 2 X 512 340 0 + 6 7 1 - 3 2 9 ;

0 + 3 X 120-19 353 0 + 3 X 1 2 0 ;

0 + 5 1 2 - 1 5 9 365 0 + 9 5 3 - 3 8 0 - 8 375 0 + 9 5 3 - 5 8 0 389 0 + 9 5 3 - 5 8 0 424 0 + 2 X 2 0 9 444 0 + 5 1 2 - 3 0 - 1 9 453 0 + 2 ) < 6 7 1 - 8 9 1 464 0 + 9 5 3 - 1 0 1 0 + 5 1 2 494 0 + 5 1 2 - 1 9 504 0 + 5 1 2 - 8 512 0 + 5 1 2 525 0 + 5 1 2 + 1 5 ;

0 + 8 5 2 - 3 2 9 552 0 + 5 6 0 - 8 560 0 + 5 6 0 572 0 + 5 6 0 + 1 5 ;

0 + 5 1 2 + 6 2 590 0 + 5 9 0 602 0 + 9 5 3 - 3 2 9 - 1 9 613 0 + 9 5 3 - 3 2 9 - 8 617 0 + 6 7 1 - 5 6 629 0 + 9 5 3 - 3 2 9 638 0 + 6 7 1 - 3 0 652 0 + 6 7 1 - 1 9 663 0 + 6 7 1 - 8 ; 0+819--159 671 0 + 6 7 1 684 0 + 6 7 1 + 1 5 707 0 + 8 1 9 - 2 ) < 5 6 712 0 + 3 ) < 5 1 2 - 8 2 2 732 0 + 2 X 9 5 3 - 1 0 1 0 - 1 5 9 746 0 + 8 1 9 - 5 6 - 1 9 752 0 + 8 1 9 - 5 6 - 8 768 0 + 8 1 9 - 5 6 774 0 + 8 1 9 - 3 0 - 1 9 788 0 + 8 1 9 - 3 0 801 0 + 8 1 9 - 1 9 ;

0 + 9 5 3 - 1 5 9 811 0 + 8 1 9 - 8 819 0 + 8 1 9 831 0 + 8 1 9 + 1 5 840 0 + 9 5 3 - 2 X 56 845 0 + 8 5 2 - 8 852 0 + 8 5 2 865 0 + 8 5 2 + 1 5 ;

0 + 2 ) < 5 1 2 - 1 5 9 896 0 + 2 ) < 9 5 3 - 1 0 1 0 906 0+953- -44 935 0 + 9 3 5 ; 0 + 9 5 3 - 1 9 945 0 + 9 5 3 - 8 953 0 + 9 5 3

APPLIED SPECTROSCOPY 285

Table I. (Continued)

Differ- Fro- enee quen- from cies O-O

Relative • intensity (in band 15°C 20°C - 1 0 ° C cm-0 (cm -1) Assignments

2 35 918 968 0 + 9 5 3 + 1 5 2 35 925 975 0+852-120;

0+2X512-19-30 1.5 35 941 991 0 + 2 X 5 1 2 - 2 X 1 9 2.5 35 958 1008 0+2X512-19 3 35 966 1016 0+2X671-329;

0 + 2 X953-891 3 35 971 1021 0 + 2 X 5 1 2 - 8 5 35 978 1028 0+2X512 4 35 984 1034 0+953+120-2X19 3 36 001 1051 0+953+120-19 9 36 029 1073 0+953+120 ;

0+560+512 9 36 031 1081 0 + 9 5 3 + 1 2 0 + 1 5 2 36 068 1112 0 + 2 X 5 6 0 - 6 2.5 36 069 1119 0 + 2 X 5 6 0 5 36 104 1154 0+209+953-8 7 36 117 1167 0+209+953 5 36 171 1221 0+1953+852-580 4 36 185 1235 0+671+560 5 36 199 1249 0 + 9 5 3 + 5 1 2 -

329+129 3 36 226 1276 0 + 9 5 3 + 3 X 1 2 0 -

2X19 3 36 246 1296 0+952+3X120-19 6 36 265 1315 0 + 9 5 3 + 3 X 1 2 0 2 36 285 1335 0+2X671-8 2 36 295 1345 0+2X671 2 36 308 1354 0 + 2 X 6 7 1 + 1 5 2 36 333 1383 0+852+560-30 2 36 351 1401 0+852+560-8 2 36 363 1413 0+852+560 3 36 376 1426 0+852+560-15 ;

0 + 9 5 3 + 5 1 2 + 1 2 0 - 159

8 36 414 1464 0+953+512

Differ- Fre- ence quen- from cies O-O

Relative • intensity (in band 15°C 20°C - 1 0 ° C cm-1) (cm-1) Assignments

3 36 424 1474 0+2X819-159-8 3 36 429 1479 0+2X819-159 2 36 437 1487 0+953+560-30 5 36 466 1516 0+953+560 3 36 477 1527 0 + 3 X 5 1 2 - 8 3 36 486 1536 0 +3 X 5 1 2 1.5 36 501 1551 0+3X512-15 2.5 36 533 1543 0+953+512-56 2.5 36 545 1595 0X2+819-30-8 2.5 36 554 1604 0+2X819-30 2.5 36 569 1619 0+2X819-19 3 36 583 1633 0 + 2 X 8 1 9 - 8 3 36 590 1640 0 + 2 X 8 1 9 2 36 603 1653 0 + 2 X 8 1 9 + 1 5 2 36 619 1669 0 + 3 X 5 6 0 - 8 2 36 626 1676 0 +3 X 5 6 0 2 36 662 1712 0 + 1 2 0 + 2 N 8 1 9 -

30-19 2 36 681 1731 0+120+2X819-30 1 36 697 1747 0+2X953-159 2.5 36 708 1758 0+120+2M819 2.5 36 720 1770 0 + 1 2 0 + 2 X 8 1 9 + 1 5 2 36 794 1844 0+2X953-56 2 36 807 1857 0+2X953-44 3 36 846 1896 0 + 2 X 9 5 3 - 8 3 36 854 1904 0 +2 X 9 5 3 1 36 892 1942 0+1958-19 2 36 908 1958 0 + 2 X 9 5 5 + 6 2 1 36 915 1965 0 + 8 5 2 + 2 X 5 6 0 ;

0+953+2X512-19 1 36 936 1986 0 + 9 5 3 + 2 X 5 1 2 2 36 946 1995 0+4X512-56 1 36 977 2027 0+4X512-19 1 36 990 2040 0 + 4 X 5 1 2 - 8 2 37 000 2050 0 + 4 X 5 1 2

Maximum intensity is 10.

C. Nonttotally Symmetrical Fundamentals

I. Nonplanar Ring Deformations

The excited-state 2O9-cm -t f requency plays a prom- inent role in the present spectrum. The 370-cm -1 ground-state f requency :has been correlated to it, and assigned to the mode (b~ type), which is assigned in pyridine-H5 to the 405 cm -1 ground-state and the 241- c m -~ excited-state frequencies. In the corresponding benzene spectrum 9 the corresponding frequencies are 404 and 240 cm -1 in the ground and excited states, respectively. This vibrat ion is held responsible for a difference band at 0-160 cm -t for benzene, at 0-164 cm -1 for pyridine-H5 and at 0-159 cm -I for pyridine-Ds.

Another vibrat ion of this type (a~ symmetry) , has been assigned to the 697-cm -~ ground-state and the 671-cm -t excited state frequencies, showing a lowering of about 27% in both the states, upon deuteration. This large frequency drop may be explained by assum- ing large motions of H atoms in this mode, due to interaction between the; skeletal modes. 1° However, the assignment of the 697-cm -1 frequency is not quite certain, as it may be explained as a combination band also.

2. Planar Ring Deformations

A ground-state frequency of 622 cm -1 appears in a medium weak band which may be taken as corre- sponding to the 625-cm -~ Raman frequency. This is identified as the planar ring deformation, correlated to the 560-cm -~ excited-state frequency. Thus a fre- quency lowering of about 4% is observed due to deuterat ion (Table II). These two frequencies form combination bands with frequencies, analogous to the frequencies of pyridine-H5 with which the correspond- ing frequencies of 652 cm -1 in the ground state and 581 cm -1 in the excited state, form combination bands ia tha t spectrum. This 652 cm -~ frequency has been assigned to two modes by Corrison et al. , t a ring- puckering, and the nontota l ly symmetric component (b2) of the 606 cm -~ vibrat ion of benzene. Sponer has preferred the lat ter assignment, 6 considering the excited-state intensi ty and the drop in its value on electronic excitation. In the present case, the strong intensi ty of the 560-cm -~ frequency strengthens the assignments. This part of the spectrum may be ex- plained as the forbidden one, due to the memory of the ~- electrons of their D6h symmet ry in benzene.

286 Volume 26, Number 2, 1972

Table II. Fundamental vibrational frequencies of pyridine-D5 (cm-1).

Pyridine-H5

Ra- Infra- Vibrational Modes man red Absorption

Ground state Excited state

Pyridine-D5 Pyridine-H5 Pyridine-D5 Low- Low

Ra- Infra- ering Absorp- ering- man red Absorption (%) Absorption tion (%)

al Type C-C-C i.p. ~ bending 605 604 601(s) b Ring breathing 992 991 992 (vw) d Triagonal bending 1029 1030 1031 (row) f C-H i.p. a bending 1214 1217 1218 (w) ~ C-H i.p. a bending 1049 1068 1063 (w) e

bl Type C-C-C i.p. a bending 652 653 649 (m) h

a2 Type C-C-C o.p i bending 886 883 891 (w) ~ C-C-C o.p. i bending 374 374 378 (vw) d

bl TYiPe C-C-C o.p." bending 404 405 405 (vw) d

582 582 580 (vs) ¢ 4 542 (S) b 512 (s) b 5.5 962 963 3 968 (w) e 935 (vw) d 3

1000 1011 1010 (s) b 3 995 (ms)g 953 (vs) c 3.3 886 886 891 (mw) ~ 27.3 1184 (ms)g 852 (vs) ~ 28 824 823 822 (w) ° 23 1017 (mw) f 819 (m) h 23

625 625 622 (w) ~ 4 581 (vw) d 560 (m) h 3.7

890 697 (VW) d 23 864( )h 871 (m) h 23 329 329 392 (mw) f 12

371 371 370 (row) f 7.5 241 (vw) d 289 (mw) f 7.5

a ip =in p l ane . b S = s t r o n g . e vs =very strong. d VW ~ v e r y w e a k . e w = w e a k . f m w = m e d i u m w e a k .

m s = m e c i u m s t r o n g . h m = m e d i u m . i op = o u t of p l ane .

D. The Shape of the Molecule in the Excited State

The presen t s t u d y of the v ibronic s p e c t r u m of pyr i - dine-D5 u n d e r low reso lu t ion m a y be used to specula te on the changes in the shape and size of the molecule on electronic exci ta t ion. We observe t h a t all the v ib ra - t iona l f requencies decrease on electronic exci ta t ion, and this po in ts to a weaker molecular b o n d i n g in the excited state. The ve ry smal l change in the v ib r a t i ons invo lv ing C a toms shows t h a t there is on ly a sl ight expans ion in the r ing size in the excited electronic state. This is fu r the r suppor t ed by the ve ry smal l re- deg rada t ion of all the p r o m i n e n t v ib ron ic bands .

A C K N O W L E D G M E N T S

The au thors t h a n k Professor N. L. S ingh for his k ind in te res t in the work. The senior a u t h o r also t h a n k s

The N a t i o n a l B u r e a u of S t a n d a r d s PL-480 fund for f inancia l help.

1. L. Corrison, B. J. Fox, and R. C. Lord, J. Chem. Phys. 21, 1170 (1953).

2. D. A. Long and W. 0. George, Spectrochim. Acta 19, 1777 (1963).

3. F. A. Anderson, Borge Bak, S. Brodsen, and J. Rastrup- Anderson, J. Chem. Phys. 23, 1047 (1955).

4. V. V. Henri and. P. Angenot, J. Chem. Phys. 33, 641 (1936).

5. H. Sponer, Rev. Mod. Phys. 16, 224 (1944). 6. H. Sponer and H. Stucklen, J. Chem. Phys. 14, 101 (1946). 7. H. Sponer and S. H. Wollman, J. Chem. Phys. 9, 816 (1941). 8. C. W. F. Spiers and J. P. Wibant, Rec. Trav. Chim. Pays.

Bas. 56, 573 (1957). 9. H. Sponer and D. S. Lowe, J. Opt. Soc. Amer. 39, 840

(1949). 10. R. K. Asundi, K. S. Krishnan Memorial Lecture (N.I.S.I.),

1970.

APPLIED SPECTROSCOPY 287