The 1,2,4-Triazolyl Cation
J . Org. Chem., Vol. 66, No. 4, 2001 1251
unscaled CASSCF(10,8) frequencies were used to compute
the zero-point vibrational energy (ZPVE) corrections to the
energies.
excitation beam served as the probe light. The probe beam
emerging from the sample was focused on the entrance slit of
an Oriel model 772.50 monochromator equipped with a model
77298 (500 nm blaze) grating in the spectral range between
300 and 800 nm. The monochromatic light was detected by a
Hamamatsu model R-928 photomultiplier tube. The timing
sequence of the excitation and probing of the sample was
controlled by a Kinetics Instruments sequence generator and
laser controller. Data acquisition and digitization were per-
formed with a Tektronix model 7104 oscilloscope in conjunction
with a Tektronix model C101 video camera and DCS-01
software. The ns/us spectral decays were processed with the
ASYST (2.0) scientific software package.26b
To incorporate the effect of dynamical valence-electron
correlation on the relative energy ordering of the low-lying
electronic states of 1, second-order multiconfigurational per-
turbation theory calculations based on the CASSCF(10,8)
reference function (CASPT2)45 were carried out. CASPT2
single point energies were calculated at the CASSCF(10,8)/
6-31G(d) optimized geometries, using the Dunning correlation-
consistent polarized valence triple-ú [10s5p2d1f/4s3p2d1f]
basis set for carbon and nitrogen, and [5s2p1d/3s2p1d] for
hydrogen, designated cc-pVTZ,46 and all valence electrons were
correlated. Since the “normal” CASPT2 method, sometimes
denoted CASPT2-0, is known to underestimate the energy of
some open-shell relative to closed-shell electronic states, the
CASPT2-g1 procedure with the full Hartree-Fock matrix was
used in the construction of the zeroth-order Hamiltonian.47 The
CASPT2 calculations were performed with the MOLCAS 4
program package.48
La ser F la sh P h otolysis. A mixture of pyridinium salt and
mesitylene was dissolved in acetonitrile (distilled from P2O5,
stored under argon, and delivered with the aid of a syringe
into a 10 mm quartz fluorimeter cell equipped with a Tef-
lon stopcock. The UV-vis spectrum of the mixture was re-
corded before laser photolysis to ensure purity prior to laser
photolysis.
F lu or escen ce Exp er im en ts. Steady-state luminescence
measurements were performed on a Spex Fluorolog-2 photon-
counting emission spectrometer equipped with a 450-W xenon
source and an R928 photomultiplier tube as detector.
X-r a y Cr ysta llogr a p h ic An a lysis. Intensity data were
measured at 22 ( 1 °C by using ω/2θ scans (2θmax ) 45°) with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) on
a Nicolet R3mV diffractometer. Periodic measurement of three
intensity standards indicated no need for a decay correction,
and analysis of azimuthal scans of several moderately intense
reflections verified that an absorption correction was not
required. Lorentz and polarization corrections were applied
to the data. The structure was solved by using direct methods
and refined by using full-matrix least-squares techniques. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed in optimized positions (dC-H ) 0.96 Å) and
were allowed to ride on the atom to which they were bonded;
isotropic group thermal parameters were refined for all of the
hydrogen atoms (Uiso(H): 0.103(7) Å2). Structure solution,
refinement and the calculation of derived results were per-
formed with the SHELXTL-Plus49 package of computer pro-
grams. Neutral atom scattering factors were those of Cromer
and Waber,50 and the real and imaginary anomalous dispersion
corrections were those of Cromer.51
Time-resolved difference absorption spectra on the picosec-
ond time-scale were obtained by utilizing the 355 nm (third
harmonic) pulse from a Quantel YG 501-C mode-locked Nd3+
:
YAG laser as the excitation source. The analyzing light was
generated by focusing the residual fundamental (1064 nm) on
a mixture of H2O and D2O (1:1 v:v) contained in a 10 cm
cuvette. The emergent white light was focused on a bifurcated
fiber-bundle (Dolan-J enner) which directed the two analyzing
beams through the excited and unexcited volumes of the
sample at a 90° angle to the excitation beam. The two
analyzing beams were collected by a fiber-optic cable and
passed through a monochromator (ISA model HR-320) to a
dual diode array (Princeton Instruments model DD-512) to
record the signal. The monochromator was calibrated with the
436 and 542 nm lines from a mercury lamp. Time resolution
was achieved by passing the fundamental (1064 nm) along a
variable-delay stage (Velmex model B4036Q13). Before each
spectral acquisition, a background data set was collected
without exciting the sample. Absorbances were calculated
using the relationship: ∆A ) IoIb/IobI, where Io represents the
intensity of the analyzing beam passing through the unexcited
Crystal data: C10H12N4, fw 188.23 u, orthorhombic space
group, Pnma (No. 62), a ) 15.780(6), b ) 7.375(3), c ) 8.806-
(3) Å, V ) 1024.8(6) Å2, Z ) 4, Fcalcd ) 1.22 g cm-3. The
trimethylpyridinium ring sits on a crystallographic mirror (x,
_, z) which bisects the triazole ring. Final residual values of R
) 0.046 and Rw ) 0.059 for 438 observed data (I > 3σ(I)).
Ack n ow led gm en t. This research was supported in
part by the Spanish DGICYT (Grant PB98-1240-C02-
01) and the Catalonian CIRIT (Grant 1999SGR-00043).
HHG is grateful for the support from Austin College and
The Robert A. Welch Foundation. A grant from the
National Science Foundation (CHE-9012972) to R.A.A.
is also gratefully acknowledged. We thank Reilly In-
dustries for the gift of 4-amino-1,2,4-triazole. Dr. Mike
Bockman carried out the laser flash photolysis experi-
ments at the University of Houston under the direction
of Professor J ay Kochi. Prof. Sun YaPing (Clemson)
carried out the fluorescence studies. Their assistance
is gratefully acknowledged.
column of the sample and Io and Ib represent the intensities
b
of the background. Spectra were acquired at
a uniform
excitation energy of 6 mJ per pulse, and they represented the
average of 300 transients.
Time-resolved spectra on the nanosecond/microsecond time
scale were obtained using the third harmonic (355 nm) of a
Quantel YG 580-10 Q-switched Nd3+:YAG laser with a 10 ns
pulse width as the excitation source. The output of a 150 W
xenon arc lamp focused on the sample at a 90° angle to the
Su p p or tin g In for m a tion Ava ila ble: Complete listings
of crystallographic data, atomic coordinates, bonding distances
and angles, anisotropic thermal parameters, packing dia-
grams, and observed and calculated structure factors. Experi-
mental procedures and full characterization for all new
compounds (1H and 13C NMR, IR, MS, elemental analyses,
mp). This material is available free of charge via the Internet
at http://pubs.acs.org.
(44) Frisch, M. J .; Trucks G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T. A.; Petersson,
G. A.; Montgomery, J . M.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, A.; Nanayakkara, A.; Challacomb, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . J . P.; Head-Gordon, M.; Gaonzales, C.; Pople, J . A.
GAUSSIAN 94; Gaussian Inc.: Pittsburgh, PA, 1995.
(45) (a) Anderson, K.; Malmqvist, P.-A.; Roos, B. O.; Sadlej, A. J .;
Wolinski, K.; J . Chem. Phys. 1990, 94, 5483. (b) Anderson, K.;
Malmqvist, P.-A.; Roos, B. O. J . Chem. Phys. 1992, 96, 1218.
(46) Dunning, T. H. J . Chem. Phys. 1995, 90, 1007.
J O001382U
(49) Sheldrick, G. M. SHELXTL-Plus (Version 4.2), Crystallographic
Computing System; Nicolet Instruments Division: Madison, WI, 1986.
(50) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography, Vol. IV; Table 2.2B, The Kynoch Press: Birmingham
England, 1974.
(51) Cromer, D. T. International Tables for X-ray Crystallography,
Vol. IV; Table 2.3.1, The Kynoch Press: Birmingham England, 1974.
(47) Anderson, K. Theor. Chim. Acta 1995, 91, 31.
(48) Anderson, K.; Blomberg, M. R. A.; Fu¨lscher, M. P.; Karlstro¨m,
G.; Lindh, R.; Malmqvist, P.-Å.; Neogra´dy, P.: Olsen, J .; Roos, B. O.;
Sadlej, A. J .; Schu¨ltz, M.; Seijo, L.; Serrano-Andre´s, L.; Siegbahn, P.
E.; Widmark, P.-O. MOLCAS version 4.1; Lund University, Sweden,
1997.