Isothiocyanate radical produced by electron impact on methyl

Elisabeth A. Wade and Jennifer L. Pore , David L. Osborn. The Journal of Physical Chemistry A ... Mark Fitzgerald and John H. Bowie. The Journal of Ph...
0 downloads 0 Views 629KB Size
Download PDF
3485

J. Phys. Chem. 1990, 94, 3485-3489

NCS Radical Produced by Electron Impact on CH,NCS and CH,SCN Ikuo Tokue,* Katsuyoshi Kobayashi: Tomohisa Honda, and Yoshio Ito Department of Chemistry, Faculty of Science, Niigata University, Niigata 950- 21, Japan (Received: July 12, 1989; In Final Form: October 18, 1989)

Emission spectra of NCS produced by low-energy electron impact on CH3NCS and CH3SCN have been investigated. The onsets forming NCS(A) from CH3NCS and CH3SCN were obtained to be 7.5 f 1.0 and 6.8 f 0.5_eV, respectively. It is concluded that NCS(A) is formed via an optically allowed process, CH,NCS or CH3SCN CH3(X2A,”) + NCS(A2n). Moreover, the second threshold near IO eV was observed from both parents. Decay curves of the NCS emission in the 360-443-nm region can be represented by three components. Fluorescence lifetimes for the fast decaying component are measured to be 168 f 6 ns for CH3NCS and 175 f 1 1 ns for CH3SCN. These lifetimes are attributed to the fluorescence lifetime of the perturbed A211 state, while the middle decaying component with the lifetime of about 600 ns can be ascribed to the fluorescence lifetime of the B2Z+ state.

-

Introduction A system of weak emission bands in the region 375-485 nm was first obtained during flash photolysis of methyl isothiocyanate (CH,NCS) and methyl thiocyanate (CH,SCN) and a high-frequency discharge through CH3SCN and was assigned to the excited N C S radical.’ Dixon and Ramsey2 analyzed the absorption bands of N C S between 330 and 400 nm and assigned them to two electronic band systems, A211i-X211iand B2B+-X211i. Later, Ohtoshi et aL3 reported the near-ultraviolet bands of N C S by laser-induced fluorescence (LIF) from the A211(000) state and assigned several transitions perturbed by Fermi interaction. Recently, Northrup and Sears4 have described high-resolution LIF studying the A-X and B-X band system under free jet expansion conditions and determined vibrational and spin-orbit constants. Although spectroscopic data of the X, A, and B states of N C S are thus available, there has been only a little amount of information about the geometrical parameters of NCS. In the present study, ab initio calculations have been carried out in order to obtain the optimized geometries and the harmonic vibrational frequencies of the X211 and B2Z+ states of NCS. D’amario et al.5 studied the photolysis of C H 3 N C S and CH,SCN in the vacuum ultraviolet (vacuum UV) region and determined the bond dissociation energy for the production of NCS(A2n) and the enthalpy of formation of NCS. Dissociation processes forming the electronically excited NCS from CH3SCN C H 3 + NCS(A,B). Production are concluded to be CH,SCN of CN( B2Z+)has been also observed in the case of CH3SCN. On the other hand, the decomposition of CH3NCS and CH3SCN and subsequent reaction kinetics of the fragments were studied by kinetic spectroscopy employed with the radiofrequency pulse discharge.6 It was concluded that the primary dissociation steps forming N C S are CH3SCN C H 2 + H N C S and CH,SCN C H + H 2 + NCS. The fluorescence lifetime of the 000 level of the A211state has been measured to be 164 f 10 and 160 f 5 ns by LIF.,q4 The kinetic information thus obtained is, however, still insufficient to discuss the formation mechanism of the electronically excited NCS radicals from CH3NCS and CH3SCN. The bond length of N C S skeleton in the ground state of CH3NCS determined by gas electron diffraction’ is fairly different from that of CH3SCN determined by microwave spectrum.8 On the basis of an application of the Franck-Condon principle, it is expected that this difference has direct effects upon the dynamics for formation of NCS. In the vacuum-UV photolysis using synchrotron r a d i a t i ~ nhowever, ,~ the NCS emission spectra from CH3NCS have been very similar to those from CH,SCN. In order to study the formation of electronically excited NCS radicals, we have observed the emission spectrum, the fluorescence lifetime, and the excitation function of the N C S emission produced by low-energy electron impact on CH3NCS and CH,SCN. Comparing the equilibrium geometries of the X2nand B2Z+ states

-

-

-

+

‘Present address: Fujitsu. Inc., Nakahara-ku, Kawasaki 21 1 Japan.

0022-3654/90/2094-3485$02.50/0

of NCS obtained by a b initio calculations with the N-C-S frame of parent molecules, we discuss the dissociation dynamics.

Experimental Section The experimental arrangement has been described previously.1° In brief, electrons accelerated and focused by three electrodes were introduced into a collision cell through an aperture. For the measurement of fluorescence lifetimes a pulsed electron beam was used with a repetition frequency of 31 kHz and a pulse width of 200 ns. The decay curve was obtained by a single photon detection method. The pressure of the sample vapor fed into the collision cell was controlled to 10-32 mPa during the emission measurement. A spectral band-pass of 1.0 nm fwhm was used for the measurement of the emission spectra and the excitation function, while 10 nm fwhm resolution was used for the lifetime measurement because of weak intensity. CH3NCS and CH3SCN (Kanto Chem.) were vacuum-distilled several times to remove CS2 and other impurities. Computational Method Understanding of the dynamics forming N C S will require parallel theoretical efforts to clarify properties of the ground and excited states of N C S and parent molecules. Since these calculations are laborious and expensive, important features of the X211 and B2Z+states of N C S have only been calculated in the present study. All computations were carried out within the framework of the GAUSSIAN 82 systemll using the basis sets incorporated in this system. The standard 4-31G* basis set was mainly employed in the calculation; this was 49 basis sets consisting of 94 contracted Gaussian functions. Self-consistent-field (SCF) calculations were made within the unrestricted Hartree-Fock (UHF) method. Stationary points on the potential energy surfaces at the S C F level were obtained by using an analytic energy gradient method. At ( I ) Holland, R.; Style, D. W. G.; Dixon, R. N.; Ramsay, D. A. Nature 1958, 182, 336.

( 2 ) Dixon, R. N.; Ramsay, D. A . Can. J . Phys. 1968, 46, 2619. (3) Ohtoshi, H.; Tsukiyama, K.; Yanagibori, A,; Shibuya, K.; Obi, K.; Tanaka, 1. Chem. Phys. Letr. 1984, 1 1 1 , 136. (4) Northrup, F. J.; Sears, T.J. J . Chem. Phys. 1989, 91, 762. (5) D’amario, P.; Stefano, G . Di; Lenzi, M.; Mele, A . J . Chem. SOC., Faraday Trans. I 1972, 68, 940. (6) Nicholas, J. E.; Amodio, C. A . J . Chem. Soc.. Faraday Trans. I 1980, 76, 1669. ( 7 ) Anderson, D. W. W.; Rankin, D. W. H.; Robertson, A. J . Mol. Struct. 1972, 14, 385. (8) Dreizler, H.; Rudolph, H . D.; Schleser, H. 2.Naturforsch. 1970,254 1643. (9) Tokue, 1.; Hiraya, A.; Shobatake, K. Chem. Phys. 1987, 117, 315. (IO) Tokue, 1.; Ito, Y . Chem. Phys. 1988, 125, 347. (11) Binkley, J. S.; Frisch, M. J.; Defrees, D. J.; Raghavachari, K.; Whiteside, R. A.; Schlegel, H . B.; Pople, J. A . GAUSSIAN-82; Carnegie-Mellon Chemistry Publishing Unit: Pittsburgh, PA, 1984.

0 1990 American Chemical Society

3486

Tokue et al.

The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 I "

(a)CH3SCN

i

l i 350

lo

1

\

200

0

ooo-ooo

\

l8Ons 550ns

1000

800

CHANNEL NUMBER

&OO

450 Ahm)

Figure 1. NCS emission spectra in the 330-490-11111region produced by low-energy electron impact measured with 1.0 nm fwhm resolution: (a) CH3SCN obtained at an impact energy of 1 1 . I eV; (b) CH,NCS at 11.9 eV.

the stationary point, analytic S C F second derivatives12*13 were computed and transformed into force constants for normal coordinates to give harmonic vibrational frequencies and normalmode eigenvectors. Final energies and equilibrium geometries of the X and B states of N C S were obtained by executing configuration interaction calculations with all single and double substitutions (CISD) from the H F reference within the frozen core approximation.

Figure 2. Typical decay curves (dots) obtained by 15-eV electron impact on CH3SCN with 10 nm fwhm resolution: (a) at 373 nm; (b) at 408 nm. Three lines show the lifetime and intensity of three decaying components.

'

r/ns

I

I

I

energy of impinging electrons have been assigned to the hydrogen Balmer lines, and the CH(A2A-X211), CN(B2Z+-X2Z+), CS(a311-X1Z+),and CS(A'II-X'Z+) bands.I4-l6 A decrease of the impact energy reduces these sharp emissions except the CS(A-X) band from CH3NCS but increases a broad band appearing in the 330-490-nm region, which is attributed to the A2ni-X211i and B2Z+-X211ibands of N C S Z Figure 1 shows the emission spectra in the 330490-nm region produced by low-energy electron impact on CH3NCS and CH3SCN. According to the l i t e r a t ~ r e , vi~-~ brational transitions are partly assigned. This assignment is still inadequate since no transition between higher vibrational levels is assigned. The emission spectra from both molecules are very similar except the emission near 380 nm from CH3SCN which are overlapped with the CN(B-X) band. Fluorescence Lifetimes. When sample gases are irradiated with the pulsed-electron beam, the population of the excited states decays freely. The decay curves of the N C S emission were measured in the 360-443-nm region. Almost all data were obtained at an electron energy as low as possible to avoid overlapping with the CN(B-X) band. Figure 2 shows typical time-dependent profiles obtained by low-energy electron impact on CH,SCN. These curves are apparently nonexponential but can be successfully analyzed as a superposition of three decaying components. (12) Pople, J . A.; Krishnan, R.; Schlegel. H. B.; Binkley, J . S. (nf. J . Quantum Chem. Symp. 1979, 13, 225. (13) Saxe, P.; Yamaguchi, Y.; Schaefer 111, H. F. J . Chem. Phys. 1982, 77, 5647. (14) Wallace, L. Astrophys. J . 1962, 7 , 165. (15) Pearse, R. W. E.; Gaydon, A. G. The Identification of Molecular Spectra, 3rd ed.;Chapman & Hall: London, 1963. (16) Taylor, G.W.; Setser, D. W.; Coxon, J . A. J . Mol. Spectrosc. 1972,

I

'

I

I

I

CH3NCS I

I

o3

1

Results Emission Spectra. Many sharp bands observed at a higher

44. 108.

600

400

i

1021

I I I I ( I I I I I I I I 350

.

400 450 WAVE LENGTH1nm

Figure 3. Fluorescence lifetimes of three decaying components obtained from CH3NCS: (0)at 16 mPa; ( A ) at 32 mPa.

The decay curves were analyzed by a nonlinear least-squares fitting procedure. After the cutoff of excitation ( t > 0), the time dependence of the emission intensity I(i,t) at the wavelength i is assumed to be represented by the following fitting function: 3

I(i,t) = Co(i)+ ECk(i) e x p [ - t / ~ ~ ( i ) ] k=l

(1)

Here, Co(i)is a background constant; C k ( i ) and T k ( i ) are the constants related to the initial intensity and the fluorescence lifetime, respectively, for the k component. The fitting function was convoluted with the time profile of the detection system. The fluorescence lifetimes and the intensities for three decaying components are shown by three lines in Figure 2. The fluorescence lifetimes thus evaluated did not depend on the sample pressure in the 10-32 mPa region and are shown in Figure 3 for CH3NCS and Figure 4 for CH3SCN. Lifetimes for three decaying components obtained from CH3NCS were constant in the 378-443-11111 region; the fluorescence lifetimes (Q, k = 1-3) with their relative intensities (Cksk) are evaluated to be 168 6 ns (8-19%), 600 f 100 ns (4-7%), and 9450 f 500 ns (74-88%). For CH3SCN, the fluorescence lifetime for the fast decaying component is 175 f 1 1 ns (31-35%), whereas lifetimes for two slower decaying components, 500-1200 ns (5-1 1%) and 42OC-7600 ns (56-64%), were found to depend on the wavelength measured in the 358-443-nm region.

*

I:;p

The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3487

N C S Radical from CH3NCS and CH3SCN

TABLE I: Predicted Equilibrium Geometries for the X2n,A211, and B2E+ States of NCS, Using 4-31G* Basis Set E," r(N-C), r(C-S), I?,, B(000),b state method hartrees A A cm-' cm-1 X2II

UHF CiSD estC UHF CISD

A2II

B2Zc CH,NCS CHjSCN

-489.2986 -489.6873

-489.1711 -489.5149

obsd obs'

"Total energies. bReference 2.

1.158 1.154 1.22 1.166 1.178 1.192 1.170

1.641 1.650 1.68 1.584 1.578 1.597 1.684

0.2050 0.2041 0.1917 0.2135 0.2130

0.2036 0.1906 0.1969

'See text. dReference 7. 'Reference 8.

TABLE 11: Vibrational Frequencies (cm-') of the X211, AzII, and B2E+ States of NCS 102

400

350

450

state

modes

calc"

scaledb

obs

X2II

VI

2046 416d 765

1920 390d 718

1 942c 387c 7OOc 192Ic 37ae 77Ic

2262 519 878

2123 543 824

V2

WAVELE NGTH/nm

Figure 4. Same as Figure 3 but for C H I S C N : ( 0 )at 16 mPa; (A)at 32 mPa.

V3

A2II

VI V2

u3

I

1

(o)CH3SCN

B2Z+

VI V2

VI

343e

"The UHF/4-31G1 results. bThe scaled frequencies have been reduced by 6.1%. cReference 4. dThe mean of two components. Reference 2.

7.0

I

i

*.,:

+a,*-

(b)CH3 NCS 10

20 30 ELECT RON ENERGY(eV1

Figure 5. NCS emission intensities at 403 and 443 nm versus electron energy measured with 1.0 nm fwhm resolution: (a) C H I S C N ; (b) CHjNCS.

Excitation Functions. To obtain the onset for the formation of the excited NCS, the excitation function of the N C S bands has been observed at wavelengths of 408 and 443 nm. The emissions at the selected wavelengths are not affected by other bands. Figure 5 shows typical excitation functions of the N C S emission at 408 and 443 nm from CH3NCS and CH3SCN. It is apparent that the profile and the onset measured at 408 nm from each parent are the same as those measured at 443 nm. This gives rise to the fast conclusion that emissions observed at the wavelength longer than 400 nm originate from the same species, i.e., the NCS(A-X) band. The N C S emission intensity from CH3NCS increases very slowly with the impact energy near the onset, while that from CH3SCN shows a steeper rise. The onsets for the N C S emission obtained are 7.5 & 1.0 eV for CH3NCS and 6.8 f 0.5 eV for CH,SCN: uncertainties mean twice the standard deviation. The value for CH,NCS is less accurate since the emission intensity near the onset was very weak. A steep rise near IO eV from both molecules corresponds to the second threshold. The energy scale for the impinging electron was calibrated by using the onset ( 1 1.03 eV) of the N2(C311,-B311,) band produced from N2.17 The excitation functions (17) Finn, T. G.;Aarts, J. F. M.; Doering, J. P. J . Chem. Phys. 1972, 56, 5632.

of the NCS(A-X) and N2(C-B) bands were measured alteratively with a mixture of N 2 and CH,NCS or CH,SCN. Molecular Structures and Vibrational Frequencies. Equilibrium geometries predicted for the X 2 n , A211,B2Z+ states of N C S are listed in Table I with the calculated rotational constants comparing with the observed rotational constants and the geometries of the N-C-S skeleton for CH3NCS and CH3SCN. Final results of molecular geometries for the X211 and B2Z+states are obtained by the CISD/4-3 lG* calculation. The geometrical parameters for the A211 state are estimated by combining bond increases given by Dixon and Ramsay2 with the present result for the X211state. They estimated the bond increases in r(N-C) and r(C-S) on excitation from the X state to the A state using the Franck-Condon principle. Vibrational frequenciesof the X 2 n and B2Z+ states are obtained by the UHF/4-3 lG* calculation. Predicted vibrational frequencies for the X and B states of N C S are compared with the observed values in Table 11.

Discussion The NCS Radical. The ground and two low-lying excited states are linear* and their occupied valence shell of the electronic configuration are known1*to be (8u)'( 2 ~ ) ~ ( 3 ~ ) X~2 ,n ( 8 ~ ) ~ ( 2 ~ ) ~ ( 9 ~ A211 )~(3a)~, ( 8 ~ ) ~ ( 2 ~ ) ~ ( 9 ~ B2ZS )~(3a)~, The 37r molecular orbital is nonbonding and largely located to the sulfur 3p, orbital (ns) with a node close to carbon atom. The 9u orbital is almost nonbonding and mostly consists of the sulfur 3p, orbital (iis), while the 2 a orbital is a N C S bonding orbital. A little amount of information is available about molecular geometries of the N C S radical. The rotational constant (E,) in equilibrium geometry of the X211state calculated from the CISD geometry is only 0.25% larger than the experimental value E(000).2 This is in a reasonable range of the difference between Be and B(000). Therefore, the CISD geometry for the ground state is (18) Walsh, A . D. J . Chem. SOC.1953, 2266.

3488 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 subjected to estimation of the geometry of the A211 state by connecting the bond increase on excitation from the X state to the A state.2 I t should be noted that Be calculated from the bond lengths thus estimated for the A state is in good agreement with B( 000). 2 On the other hand, B, of the B2Z+ state evaluated from the C l S D geometry is 8.1% larger than B(000). This discrepancy means that the predicted bond lengths are shorter than that expected from B(000). The C-S bond should be lengthened on excitation (90 37r) from the ground state to the B state since the 9u orbital has a little of oCscharacter. However, the CISD geometry for the B state is inconsistent with this description of the orbital. Moreover, the excitation energy of the B state predicted at the CISD level is 0.2 eV larger than the observed energy. The failure of the single-reference GAUSSIAN 82 calculations indicates that the B state cannot be well described by a single reference. Thus, a multireference CI treatment would be needed to correct this deficiency. Vibrational frequencies for the X, A, and B states of NCS are compared in Table 11. The calculated frequency of the bending mode ( v 2 ) for the XzII state consists of two components (457 and 375 cm-I). Since the a b initio calculations neglect spin-orbit coupling, the two frequencies are appropriate to the two 2: vibronic levels which are two Renner-Teller components of the n state. Thus, the mean (416 cm-I) should be compared to the experimental w2,(387 cm-’) and the ordered difference (-82 cm-I) with the experimental tuz (-55 f 15 cm-’).’ I t is well-known that the vibrational frequencies calculated a t the H F level are usually higher than the observed values. Thus, we find a scale factor in the case of the ground state of N C S in bringing HF/4-3 IC* harmonic frequencies into agreement with the experimental values.z The scaled frequencies have been reduced by 6.1 %. The scaled frequencies for the X state are in good agreement with the observed, whereas the scaled u2 value for the B2Z+state is 58% larger than the observed value. This indicates that the calculation in the H F level is inadequate to predict the molecular field of the B state. The NCS Emission. The X211 and A211 states of N C S show large spin-orbit constants and further complicated by the Renner-Teller interaction and the Fermi r e s ~ n a n c e . ~Moreover, -~ the B2Z+state is known to be only 802 cm-l above the A211state and these states interact with each other via vibronic couplings! Since the B states lie very close to the A state there are many levels in the two states which lie relatively close in energy or even accidentally degenerate and have the same vibronic symmetry. The heterogeneous interaction between such levels according to the Kronig selection rules1’ may be appreciable since the electron configurations for these states differ only in the promotion of an electron. Thus. many vibronic levels of the A and B states seem to be coupled. In the NCS emission shown in Figure 1 the vibrational progressions are clearly observed on the long-wavelength side of the band origin up to 470 nm. Each peak shifts to the long-wavelength side of the O O O - v l ” v ~ ’ u ~ ’ bands. Each peak probably consists of the emissions from higher vibrational levels of the A state with the same sequence since the vibrational frequencies for the A state are very nearly those for the X state. Continuous emission observed on the short-wavelength side of the band origin disappeared near 330 nm. This can be ascribed to predissociation of vibrational levels 4000 cm-’ above the lowest level of the A state. The emission spectra from both molecules show very similar vibrational structures. On the basis of the Franck-Condon principle, the NCS emission spectra from CH3NCS are expected to be different from those from CH3SCN since the N-C-S skeleton for CH3NCS is fairly different from that for CH3SCN. Therefore, the excited NCS states produced by electron impact seem to maintain no longer the geometrical properties of parent molecule. A similar result is obtained in photolysis of CH3NCS

-

Tokue et al. and CH3SCN at 153 nm using synchrotron radiation.’ Excited-State Lifetimes. The fluorescence lifetimes for the fast decaying components were evaluated to be 168 f 6 ns from CH3NCS and 175 f 1 1 ns from CH3SCN. These values are slightly longer than the published values of 165 f I O ns3 and 160 f 5 ns4 for the 000 level of the A211 state. The fluorescence lifetime for A-state vibronic levels in close proximity to B-state vibronic levels is reported to appear around 175 ns and the lifetime for the 001 level of the B state is 225 f 5 n s 4 Thus, our measured values mean lifetimes for A-state vibronic levels pcrturbed by the B state. On the other hand, the NCS(X-B) transition is a one-electron transition (9u 3n) which is primarily a 3p, 3p, transition on sulfur atom. Since this is not a dipole-allowed process, the B state certainly has a long lifetime. A similar situation is observed for NCO, where the fluorescence lifetime of the AZZ+(OOO)level, 360 ns?O is much larger than that for the B211(000) level, 63 w2’ N o literature has mentioned the middle decaying component with the lifetime of 600 ns observed from CH3NCS. For NCS, if the A state is weakly coupled with the B state, then one of the mixed states will have a lifetime slightly longer than that of the A state, while the other will have a lifetime slightly shorter than that of the B state. Since the observed lifetimes for the fast decaying component are very nearly the lifetime of the unperturbed A state, the observed lifetime of 600 ns is probably approximated to the lifetime of the B state. Thus, we prefer that the lifetime of 225 ns observed for the 001 level of the B state4 is not the true B state since even the vibrationless level of the B state is perturbed by the 020 ~~nlevel of the A state.4 Incidentally, the middle decay component cannot be attributed to other emitters such as the CN(A-X) band since the CN(A-X) emission is very weak below 430 nmI4 in spite of its fluorescence lifetime of 680 ns.2z It is difficult to identify the origin of the slow decaying component with the fluorescence lifetime of 9500 ns from CH3NCS and 4200-7600 ns from CH3SCN. These lifetimes are hardly caused by secondary processes including scattered electrons since the energies of the impinging electrons were as low as 15 eV in the measurement. There may be slow formation steps for NCS via metastable parent states below 15 eV giving a range of the measured lifetime. The intensity of the slow decaying components is evaluated to be 74-88% for CH3NCSand 56-64% for CH3SCN at the impact energy of 15 eV. This means that slow decaying processes from CH3NCS are more predominant than those from CH3SCN. In the fluorescence excitation spectra the second rise at 10 eV for CH3SCN is steeper than that for CH3NCS; accordingly, the process correlating with the second rise for CH3SCN seems to be more important than that for CH3NCS. Thus, the slow decaying component from both molecules can be ascribed to the process with the onset near 7 eV and then the faster decaying components to the process with the second threshold near I O eV. Formation ofthe Excited NCS Radicals. It is well-known that the cross section for an optically allowed excitation shows a broad maximum above the onset.23 The intensities of the NCS(A-X) band increase with the electron energy and reach a maximum as shown in Figure 5. This gives rise to that NCS(A) is produced via an optically allowed transition. Dissociation processes forming the A and B states of NCS from CH3NCS and CH3SCN which can occur at lower impact energy are as follows:

-

CH3NCS CH3NCS

-

-

CH3SCN

-

CH3(,%zA2”)+ NCS(A211), 6.69 eV (2) CH3(R2A2”)+ NCS(B2Z+), 6.79 eV (3)

-+

CH3(R2A2”)+ N C S ( A 2 n ) , 6.39 eV (4)

(20) Charlton, T. R.; Okamura, T.; Thrush, 9. A. Chem. Phys. Lerr. 1982, 89, 98.

(21) Sullivan, 9. J.;Smith, G.P.; Crosley, D. R. Chem. Phys. Lett. 1983, 96, 307.

(19) Herzberg, G.Molecular Spectra and Molecular Structure, Vol. I : Specrra of Diatomic Molecules, 2nd ed.: Van Nostrand Reinhold: New York. 1950: pp 284-286

( 2 2 ) Jeunehomme. J . Chem. Phys. 1965, 42. 4086. (23) Massey, H . S.; Burhop, E. H. S. Collision of Electrons with Atoms; Oxford University Press: London, 1969; Chapter 4.

CHjSCN

-

3489

J . Phys. Chem. 1990, 94, 3489-3494 CHj(%2Az”)

+ NCS(B22+),

6.49 eV (5)

The calculated thresholds are evaluated from the enthalpies of f o r m a t i ~ nand ~ . ~the ~ electronic energies of the A and B states of N C S 2 The onsets observed at 408 and 443 nm correspond to the NCS(A-X) emission. The observed onsets 7.5 f 1.0 eV for CH3NCS and 6.8 f 0.5 eV for CH3SCN are 0.9 and 0.5 eV, respectively, higher than the thresholds calculated for reactions forming NCS(A). On the basis of the Franck-Condon principle, the larger difference between the observed onset and the calculated threshold for CH3NCS is probably ascribed to the direct excitation into a steeper potential surface leading to dissociation. This originates in the fact that the geometry of the NCS(A) state is very different from the bond length of the N-C-S skeleton for CH,NCS.’ The difference between the onset and the threshold is available as the translational and internal energies among the fragments produced. Thus, it is expected that the vibrational structures from C H 3 N C S is more enhanced than those from CH3SCN. There is, however, no noticeable difference between the emission spectra from CH3NCS and from CH3SCN as discussed previous section. (24) Wagman. D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, 1.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Dora 1982, I / , Suppl. 2.

Concluding Remarks We have studied the NCS emission in the 330-500-nm region produced by low-energy electron impact on C H 3 N C S and CH3SCN. The vibrational progressions well developed in the 390-500-nm region are attributed to the NCS(A-X) transition. The emission below 330 nm suddenly disappears. This is probably caused by the predissociation of vibrational levels 4000 cm-’ above the lowest level of the A state. The decay curves of the N C S emission can be represented by three decaying components. The lifetimes for the fast decaying component from both parents are attributed to the perturbed NCS(A) state, while the lifetimes for the middle decaying component are probably correlated with the NCS(B) state. On the other hand, the predominant slow decaying component (4-10 ps) from both parents is probably described to slow formation steps for N C S via metastable parent states below 15 eV. This is consistent with the result that the emission spectra from both molecules show very similar vibrational structures. In slow formation steps, the available energy can be distributed among all internal states in a statistical way and then the produced N C S no longer maintains the geometrical information of the parent molecule. Acknowledgment. I.T. thanks the Computer Center, Institute for Molecular Science, for the use of the HITAC S810/M680H computer and Library Program GAUSSIAN 82.

Recovery of Fluorescence Lifetime Distributions: Application to Forster Transfer in Rigid and Viscous Mediat Brian D. Wagner and William R. Ware* Photochemistry Unit, Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 587 (Received: July 19, 1989; In Final Form: October 31, 1989)

Fluorescence lifetime distributions are recovered from a dipole-dipole energy-transfer system (donor phenanthrene, acceptor acridine) in rigid and viscous media. In the case of a rigid medium, the well-known Forster equation is verified to high precision. An exact equation for the rate constant distribution predicted by Forster theory is obtained and is shown to agree well with the experimentally recovered distribution. The value of the critical transfer distance Ro is calculated from the maximum of the recovered distribution, the result agreeing with that obtained from the fit to the Forster equation. In the case of viscous media, several popular models are compared by establishing an empirical relationship between the maximum of the recovered rate constant distribution and Ro, in a series of viscous solvents.

Introduction For dipole-dipole electronic energy transfer in condensed media there are two limiting cases. In the first case where no diffusion occurs or diffusion is unimportant, Forster kinetics are obtained. At the opposite extreme of low-viscosity solvents, ordinary Stern-Volmer kinetics are observed and the fluorescence decay of the donor follows a single exponential. The intermediate case is complex, and the useful working equations are obtained by approximate methods. In both the high-viscosity case and the intermediate case the donor decay does not follow a single exponential, and in the latter the time dependence is quite involved. In both of these cases, one intuitively expects to find a distribution of lifetimes. I n fact, it is important to be able to recognize the typical shapes of distributions that arise from Forster transfer. There have been a number of studies reported in the literature concerning the validity of Forster kinetics. Millar et a1.I showed the validity of the donor decay law from 1 ps to IO ns after excitation for a donor with an unquenched lifetime of 3 ns, and they found that the critical transfer distance R, (defined as the ‘Contribution No. 426.

0022-3654/90/2094-3489$02.50/0

donor-acceptor distance at which the probability of energy transfer is equal to t h i probability of decay of the isolated donor) was constant over a 1000-fold range of acceptor concentration. However, the donor decay curve was only collected to 3 X IO4 counts in the peak channel (CPC). This level of precision may not reveal small deviations. Struve et aL2 collected donor decay curves to IO5 CPC but only looked at donor-donor interactions. Eisenthal .et a1.j verified the donor decay law using time-resolved ground-state absorption but did not directly collect and fit the donor decay. Thus, in spite of the capability of the time-correlated single-photon technique to generate very accurate donor decay curves, a study using such decay curves does not appear to have been done at very high precision. In the course of this work, donor decay curves on the order of lo6 CPC were collected, and these were used to rigorously test the validity of the Forster donor decay law. ~

~~

~~

( I ) Millar, D. P.; Robbins, R. J.; Zewail, A. H. J . Chem. Phys. 1981, 75, 3649. (2) Hart, D. E.; Anfinrud, P. A.; Struve, W. S . J. Chem. Phys. 1987,86, 2689. (3) Rehm, D.; Eisenthal, K . B. Chem. Phys. Lett. 1971, 9, 387.

0 1990 American Chemical Society