SPECIAL ISSUE PREFACE pubs.acs.org/JPCA
Scientific Autobiography of David W. Pratt was born in Providence, RI on September 14, 1937, the first child of Barbara (Bobbie) F. Fisher and Norman (Jim) T. Pratt, both first-generation college graduates in their respective families. After periods of residence in Princeton, NJ; Washington, DC; New York, NY; and Bloomington, IN, I received my A.B. from Princeton University in 1959, served three years in the U.S. Navy in 195962, received my Ph.D. from the University of California, Berkeley in 1967, and did postdoctoral work at the University of California, Santa Barbara in 19678. I joined the faculty at the University of Pittsburgh in 1968, where I have remained during my entire academic career. Early influences in my life include many happy summers at a lake cottage in Maine, where my paternal grandfather taught me a lot about nature and working with my hands; a year at McBurney School in New York, where I learned how to survive in a competitive academic environment; and four years at Bloomington High School, where a chemistry teacher named Avis Rector stimulated my interest in science and participation in high school science fairs. At Princeton, I found strong mentors in a remarkably talented and supportive set of teachers who helped me adjust to the rigors of college work and stimulated me to think about a career in chemistry. General chemistry from Hubert Alyea and John Turkevich, analytical from Clark Bricker and Wallace McCurdy, organic from Everett Wallis and Dick Hill, inorganic from Bob Naumann, and physical from Walter Kauzmann assisted in sealing my fate, as did general physics courses with Eric Rogers (Physics for the Inquiring Mind) and John Wheeler (who invited Niels Bohr to visit class one day!). Junior and senior thesis work in McCurdy’s lab stimulated my interest in experimental techniques, and acquainted me with some of the challenges of doing original research. After graduation from Princeton in 1959, life’s lessons continued with three years’ service in the U.S. Navy. As a line officer aboard the destroyer USS Barry, I served successively as Electronics, Communications, CIC, Navigation, and Operations Officers, third in command of the ship during my final year. Travels included “Gitmo”, the Caribbean, and around Cape Horn; to the North Atlantic, with its unbelievable winter storms; and to Western Europe, with cruises in the Baltic and the Mediterranean. It was a “heady” time on the Barry: protecting U.S. interests in Cuba, evacuating U.S. citizens from the Dominican Republic, and serving as a recovery ship for the John Glenn space flight in February, 1962, not to mention the challenges aboard ship, with its 250-man crew and many technological innovations in electronics. (I first learned about microwaves from my CPO in the electronics materials division.) Still, despite Adm. Rickover’s attractive offer to join the submarine force, I elected to eschew a Navy career and go to graduate school, where the opportunities in pure research and teaching were still growing by leaps and bounds in response to the Soviet launch of Sputnik in 1957. The learning accelerated after admission to Berkeley in June, 1962, where I joined the Ph.D. program in chemistry at the University of California campus there. Coursework was a struggle after the hiatus, but Norman Phillips and David Shirley in thermo and stat mech; Dudley Herschbach and Bill Gwinn in
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quantum; Andy Streitweiser in organic; and several instructors in physics and math, including Michael Tinkham in group theory, took pity on me and passed me with flying colors. (Passing the German language exam was another story!) I joined the research group of Rollie Myers, who among other distinctions was the first to solve the microwave spectrum of an asymmetric top, and began research on the electron spin resonance spectroscopy of free radicals in solution and, later on, photoexcited triplet states and the transition metal compounds V(CO)6 and VCl4 at liquid helium temperatures. (The JahnTeller effect (JTE) became a favorite subject.) George Pimentel was the chair of my qualifying committee; other committee members included Bob Connick; Bob Harris; and a visitor to Berkeley that term, Jack Linnett. Good friends (and beer-drinking companions) in grad school included Lester Andrews, Tony Arrington, Bill Dove, Arnie Falick, David Harris, Jack Kinkade, Don Levy, Doug McCain, Joe Nibler, Brock Spencer, and Chad Tolman. (Yuan Lee worked all hours across the hall.) A great deal was going on in the world at that time. 1964 saw the formation of the Free Speech Movement, and much subsequent turmoil on (and off) campus. A crucial issue at Berkeley was whether the chemistry TA’s would strike. (While they did not, I count this as likely instrumental in influencing my activist spirit!) I published my first paper and received my Ph.D. in 1967, an auspicious beginning. A few weeks later, I moved to Goleta, California to join Herb Broida’s group in physics at the University of California, Santa Barbara (UCSB) as an NIH postdoctoral fellow. (This collaboration was catalyzed by David Harris, who had recently joined the chemistry faculty there.) Herb, like Rollie, was a great mentor for me; they both had an unparalleled enthusiasm for science, and they both worked hard to help their students and postdocs achieve at the highest level. Herb, in particular, guided me through the funding issues in science, from which I had been largely insulated at Berkeley. This led to joint attendance at several ONR and AFOSR meetings; at one ONR meeting on the UCSB campus, I still recall an intriguing presentation by Mostafa El-Sayed in which he described the observation of a multiexponential decay of the phosphorescence of pyrazine at liquid helium temperatures. (This discovery led to the hypothesis that the different electron spin sublevels of the triplet state were decaying radiatively at different rates, a hypothesis that was quickly confirmed only months later in the now classic ODMR/ PMDR experiments of Al Kwiram; I. Y. Chan, Jan Schmidt and Joan van der Waals; Mark Sharnoff; and Dino Tinti, Charles Harris, Gus Maki, and El-Sayed.) As a postdoc, I did experiments on one of Herb’s favorite molecules, CN, and was the first to show (using the double resonance scheme of Evenson and Broida) that the cross section for collisional relaxation of quantum mechanically “perturbed (or mixed) levels” of the excited B state is significantly larger than that for unperturbed levels. This is also Special Issue: David W. Pratt Festschrift Published: August 25, 2011 9330
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The Journal of Physical Chemistry A where I had my first experience with lasers, an experience that was to foreshadow much to come in my career. Because Herb Broida and Fred Kaufman were good friends, a brief job search led to my joining the University of Pittsburgh as assistant professor of chemistry in November, 1968. Pitt was, and remains today, a great place to begin an academic career. New faculty colleagues were extremely supportive, and there were lots of resources, including excellent machine, glass, and electronic shop facilities. Intrigued by the results of El-Sayed on pyrazine and inspired by my own experiences with triplet states and double resonance methods, my initial research focused on the development of high-field methods for the detection of the ODMR spectra of the lowest triplet states of organic molecules in low-temperature crystals. Over the next few years, I recruited several excellent students (Phil Brode, Rich Kashmar, Bryan Lynch, Jay Mucha, Shermila Singham, and Henry Yue) and postdocs (Robert Chen, Gopalan Kothandaraman, and Seigo Yamauchi) to join me on this project. A number of publications followed which showed that one could determine the signs, orientations, and magnitudes of the principal values of the g, zerofield splitting, quadrupole coupling, and hyperfine splitting tensors of triplet states using the high-field method. The development of an optically detected ENDOR experiment was crucial to this task. A particularly interesting example of an application of these methods was the lowest triplet state of benzophenone; here, studies of the O-17 hyperfine and quadrupole couplings in both zero- and high-field (in the single crystal host DDE, discovered by Ahmed Zewail in his graduate work at Penn) led to the discovery of “orbital rotation”. Level crossing, cross relaxation, and exciton effects also were extensively explored. Serendipity has played a huge role in my career. A chance meeting with Roger Lloyd and David Wood from CMU’s chemistry department led to extensive collaborative studies of organic free radicals in an adamantane matrix using optical and EPR techniques. These experiments showed that a wide variety of such radicals could be prepared simply by recrystallizing the solid from different solvents, pressing it into pellets, and irradiating these with UV or X-ray “light”. The resulting EPR spectra were found to be “isotropic”, as a consequence of the rapid tumbling motion of the radical within the plastic crystal matrix. At the same time, radical diffusion was inhibited, leading to long radical lifetimes, relatively high signal-to-noise, and easily interpretable spectra. Systems studied in this way included several alkyl and alkenyl radicals, alkyl amino and imino radicals, and alkanonyl radicals. The use of C-13 and O-17 labels (suggested by Ted Cohen) led to a quantitative determination of the degree of resonance stabilization in many of these systems. Several Pitt undergraduate and graduate students (Bill Beaudry, Don Camaioni, Jack Dillon, Ellen Edelheit, Jimmy Jordan, Nancy de Tannoux, Alan Tegowski, and Henry Walter) participated in this work. Other collaborations developed in the next few years with Paul Dowd (trimethylenemethane); Fred DeRubertis and Pat Craven (nitrosyl-heme and -catalase); and Lon Knight, Mike Paddon-Row, Dan Fox, John Pople, and Ken Houk (the dynamic JTE in CH4þ). Houk and his group would later contribute greatly to my understanding of the origins of methyl rotor barriers in organic molecules. I took my first sabbatical at the University of Leiden in January, 1979, after a cordial reception from Jan Schmidt and Joan van der Waals during an earlier visit. I was impressed by the outstanding facilities at the Center for the Study of Excited States in the Huygens Laboratorium as well as by the many talented faculty,
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staff, postdocs, and students in the Leiden academic community. The Dutch culture also played a role; there were many opportunities for learning, and I especially enjoyed the public’s interest in and appreciation for science at all levels. And, as expected, I had a great time working in the lab with Joan’s and Jan’s students, especially Cor Nonhof, with whom I studied triplet state crossrelaxation phenomena. But serendipity struck again, as the most important event that occurred during this year abroad was an unexpected chance meeting with Jan Kommandeur at a gathering of the Dutch Science Foundation in Lunteren. This led to a collaboration that changed my scientific career. At the time, Jan and his student Gerard ter Horst had just constructed a supersonic jet machine. They were engaged in the study of the fluorescence decay properties of p-benzoquinone (PBQ) to detect “the sparse manifold of a lower electronic state” that was thought to influence the dynamics in the isolated molecule, according to an earlier paper by Louis Brus and Jim McDonald. What then came out of a discussion between Jan and me was the thought that this lower state might be a triplet state and that this state might be detectable by an ODMR experiment in the gas phase. So a few months later, I loaded up a rental car with microwave equipment from Leiden (borrowed with permission!), drove to Groningen, and spent two weeks looking for signal, without success. But the groundwork was laid for further experiments; I returned to Groningen in the late summer of 1980 with a NATO grant and worked there for about a month. Excited singlet state PBQ was thought to be in the “smallmolecule” limit of radiationless transition theory, despite its size. Jan thought that this might be a consequence of a “cancellation” of the spinorbit coupling from its nearly degenerate T1 and T2 states, and so he proposed that Gerard and I do some experiments on pyrazine, in which the triplet state separations were thought to be larger. A day later, on Sept. 11, we discovered that pyrazine exhibits a biexponential fluorescence decay when excited by a frequency-doubled dye laser, as expected for a molecule in the “intermediate case”. (Biexponential decays of pyrazine in a bulk gas phase sample had earlier been detected by Andre Tramer and co-workers in Orsay.) My role in these experiments was to tune the doubling crystal by hand! Subsequent experiments with a pressure-tuned etalon showed that the ratio of the fast and slow components depended on the rotational level excited; at J0 = 2, a single exponential decay of ∼400 ns was observed, but at higher J0 , a fast ∼10 ns component appeared whose relative contribution increases with increasing J0 . Collisions were shown not to be important because these effects persisted up to 6 cm downstream of the nozzle. Thus, the only logical conclusion to be derived from these experiments was that the state prepared by the laser was a coherent admixture of underlying molecular eigenstates (ME’s), derived from the mixing of one zero-order singlet level with many nearly isoenergetic zero-order triplet levels, and that the number of such states depended on J0 . Despite much previous work on this problem, Gerard, Jan, and I were the first to detect a “rotational-state dependence” of radiationless transitions. Direct confirmation of this idea was provided by the measurement in Nijmegen (by van der Meer, Jonkman, Kommandeur, Meerts, and Majewski) of the ME spectrum of the (0, 0) band of the S1S0 transition of pyrazine in a molecular beam using a 200 kHz wide CW laser operating in the UV, reported in November, 1982. (Serendipity also played a role in these experiments; the fact that such a laser/beam setup existed at the University of Nijmegen was fortuitously revealed at the May, 1982 thesis 9331
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The Journal of Physical Chemistry A defense of Gerard ter Horst in Groningen. Gerard had been an undergraduate at Nijmegen, and chose Leo Meerts to be the outside examiner for his thesis defense!) The results showed that each rotationally resolved transition in the fluorescence excitation spectrum of pyrazine actually consists of many randomly spaced lines of widely varying intensity, spread over a frequency range of a few gigahertz and having individual widths of a few megahertz, whose density increased with increasing J0 . The square of the Fourier transform of this spectrum yielded the quantum beats (and later, when off-resonance effects were taken into account by Emile Medvedev, the biexponential decays) that had earlier been observed in the time domain. A “proof” of the theory of radiationless transitions was thereby established. Thus, Wilse Robinson’s suggestion that pyrazine would be a good choice as an intermediate case molecule proved to be a prescient one. Returning to Pittsburgh after these heady experiences, I attracted several new students (Jeff Johnson, Lee Spangler, and Jeff Tomer) and postdocs (Bill Chisholm, Karl Holtzclaw, and Yoshi Matsumoto) to my research group. The group built its own supersonic jet machine (with the help of drawings from Bill Flygare and Don Levy) and performed a series of gas phase experiments on triplet states. Nanosecond time-resolved experiments on pyrazine were the first to be performed, in both the presence and absence of a magnetic field. These established the triplet character of the prepared state and showed that the strong rotational state dependence of the decay had its origin in a breakdown of angular momentum selection rules. Fluorescence depolarization measurements made possible the direct observation of the prepared state into a group of phase-incoherent ME’s. The “non-ergodic” character of the triplet vibrational levels that contribute to these ME’s was established by a LawrenceKnight deconvolution procedure applied to J0 = 0 states and, with the help of Willem Siebrand and Leo Meerts, to nonzero J0 states. Similar experiments were performed on pyrimidine, whose “sparse” ME spectrum made possible an intriguing study of magnetic field effects on collision-induced ISC suggested by Karl Freed. Then, the direct pumping of triplet states in supersonic jets was reported in 1986 using a new cold-finger detector, which led to a series of experiments on glyoxal, methylglyoxal, biacetyl, pyrazine, acetophenone, and benzophenone, some with rotational (and even spin) state resolution. The benzophenone results catalyzed an enjoyable collaboration with Rick Heller and his group in Seattle during my next sabbatical there in 1986. Intrigued by the potential of rotationally resolved electronic spectroscopy for studies of the structural and dynamical properties of molecules important to chemistry, I then assembled a team of researchers to design and build a high-resolution CW ring dye laser and molecular beam machine in Pittsburgh. David Hercules, chair of the department at the time, and Jerry Rosenberg, dean of the school, provided key financial support from the university, matching an equipment grant from NSF. In-house expertise on beam machines was provided by Peter Siska, a chemistry colleague. But the lion’s share of the work was performed by Wojciech Majewski, a visiting scientist who had played a key role in the construction of the Nijmegen apparatus, and an extraordinarily capable team of graduate students (Blaise Champagne, Andy Held, Sue Humphrey, Surya Jagannathan, Jim Pazun, Jim Pfanstiel, David Plusquellic, and Xue-Qing Tan). The first spectrum recorded with the new instrument was the S1S0 fluorescence excitation spectrum of 1-fluoronaphthalene and was published in late 1989. It exhibited thousands of resolved
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lines spread out over the narrow energy interval of 23 cm1, each with a fwhm of ∼3 MHz, corresponding to a resolution of 3 108 at the probing wavelength of ∼300 nm. Rapid, computer-based techniques were then developed (chiefly by David Plusquellic and his brother Jim) for assigning each line in spectra like this, thereby transforming electronic spectroscopy into the kind of “pattern-recognition” technique that is common to high-resolution NMR. The continuing collaboration with Leo Meerts helped immensely to improve the group’s ability to interpret such spectra, which often exhibit frequency splittings or intensity anomalies that cannot be observed at lower resolution. Colleague Ken Jordan and his group have provided important theoretical support for many of the group’s projects over the years. High-resolution studies of the isoelectronic molecules 1- and 2-hydroxynaphthalene (1HN and 2HN), 1- and 2-aminonaphthalene (1AN and 2AN), and 1- and 2-methylnaphthalene (1MN and 2MN) followed and revealed the tremendous power of this technique for studying chemical problems. The hydroxynaphthalenes exhibit cis and trans isomerism. Replacement of the hydroxyl hydrogens by deuterium changes the frequency spacings of the resolved rovibronic transitions enough to make possible the determination of the COM position of the substituted atom (to within 0.02 Å) using Kraitchman’s equations, an experiment suggested by Ken Janda, and identifications of the two species in each case. The spectrum of 1AN revealed yet another surprise: its (0, 0) band was found to be a b-type band rather than the a-type band observed for other 1-substituted naphthalenes, evidencing 1Lb/1La state reversal in the isolated molecule. (Again, it is the observation of rotational structure that makes this kind of determination possible; the selection rules on J and K depend on the orientation of the electronic transition moment in the inertial frame.) In 1MN and 2MN, splittings on the order of tens of megahertz were observed in each of the individual rovibronic lines in the spectrum owing to methyl group internal rotation; detailed analyses of these and their dependence on J and K made it possible to determine the magnitudes (and signs) of the torsional barrier heights in both electronic states. The torsional potentials of the attached COOH group in 1-naphthoic acid and the CHdCH2 group in 2-vinylnaphthalene were derived from high-resolution studies of several of the FranckCondon active vibronic bands in their S1S0 spectra. Two-top motions were studied in 2,3-dimethylnaphthalene by visitors Dennis Clouthier and Richard Judge. New students (David Borst, Cheol-Hwa Kang, Tim Korter, Phil Morgan, Tri Nguyen, Diane Mitchell, Alexei Nikolaev, Jason Ribblett, and John Yi) eventually joined the group; together with some of the original members (and visitors from the groups of John Simons at Oxford and Tim Zwier at Purdue), they greatly extended studies of this type to a wide range of new molecules, including toluene and other alkylbenzenes; styrene, phenylacetylene, and tolane (diphenylacetylene); all-trans-octatetraene, trans-stilbene, and all-trans-1,4-diphenyl-1,3-butadiene; several divinylbenzenes and ethynylstyrenes; hexahydropyrene; hydroquinone, 2-hydroxypyridine/2-pyridone (2HP/2PY), and 2-hydroxyquinoline; aniline, benzonitrile, the toluidines, and 4,40 dimethylaminobenzonitrile (DMABN); anisole, dimethoxybenzene and a number of other substituted benzenes, including acids and acid esters; and indole, azaindole, and 4- and 5-imidazoles. Some highlights of this work include discovery of inertial axis rotation (“axis tilting”) in the S1S0 transition of 2-pyridone, the observation of different nuclear spin statistical weights in the 9332
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The Journal of Physical Chemistry A electronic spectra of cis- and trans-hydroquinone, and a “precessing” rotor in p-toluidine. Separately, I explored the scaling properties of molecular spectra with Yossi Klafter, in Israel, and (at low resolution) conformationally induced rotations of electronic transition moments with Paul Joireman, Romano Kroemer, and John Simons in Oxford, where I took my third sabbatical leave in 1996. My friendship with John continues to this day; he has been a tremendous source of inspiration to all of us working in the field of biological molecules in the gas phase. Being sensitive to the moments of inertia about different axes also makes it possible to measure the center-of-mass coordinates of an attached atom or group in weakly bound van der Waals or hydrogen-bonded complexes formed in the jet expansion. This method was first applied to the Ar complexes of t-stilbene (with Meerts), 1- and 2-fluoronaphthalene (1/2FN), aniline (with postdoc Wayne Sinclair), and indole and azaindole from which intermolecular potential energy surfaces of the attached atom were derived. “On-top” N2 complexes of aniline and p-difluorobenzene (pDFB) were studied by postdoc Martin Schaefer; several students studied water complexes in 2PY, indole, benzonitrile, pDFB, anisole, and dimethoxybenzene, and ammonia complexes in 2PY, hydroquinone, the hydroxynaphthalenes, and indole. In many cases, the high resolution spectra are split into two subbands by the motion of the attached “solvent” with respect to the solute molecule; application of the torsion-rotation Hamiltonian that was developed to treat the methyl rotor problem to these systems made possible the determination of the position, orientation, and barrier to motion of the complexed species in both electronic states. “Solvent reorganization” on electronic excitation was observed in some systems. For example, in pDFB/N2, the orientation of the NN axis with respect to the FF axis changes when the photon is absorbed, owing to light-induced changes in the distribution of π-electrons in the ring. In 2PYH2O, the attached water molecule forms two hydrogen bonds with the amine hydrogen and the carbonyl oxygen atoms, so its internal motion is significantly restricted. Still, the structure of the solute molecule was shown by isotopic labeling experiments to be distorted toward a zwitterionic structure on solvation by water. Ammonia acts as both a hydrogen bond donor and acceptor in 2PYNH3. Quantum interference effects were observed in the spectrum of 2HNNH3; the simultaneous existence of hybrid band character and torsion-rotation coupling opposed by a low barrier made possible the determination of the absolute orientation of the S1S0 electronic transition moment vector in the isolated molecule. Attachment of a CH4 molecule to 1/2FN produced splittings of their electronic origin bands into three distinct subbands, which were interpreted as perturbations of the normally isotropic rotational motion of methane by the “surface” to which it is attached. The adventitious discovery of the cis-peptide dimer of 2PY by Andy Held led to several applications of the high resolution technique to biologically relevant molecules. This species was first detected as a strong band in the low-resolution spectrum of 2-hydroxypyridine (2HP). The rotational structure in the highresolution spectrum of this band clearly identified the carrier as (2PY)2, the two monomer units being linked by two NH 3 3 3 O hydrogen bonds. (2PY is the keto form of 2HP.) Analyses of the dimer’s rotational constants and those of the deuterated analog made possible estimates of the hydrogen bond distances and angles in the isolated species. Similar experiments were performed on 2PY/2HP dimer and on the 2AP/2PY dimer, an
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analog of the A/T base pair, a WatsonCrick structure with NH 3 3 3 N and NH 3 3 3 O hydrogen bonds. In all cases, the gas phase values of these distances are comparable to those observed in crystals. With David Plusquellic, it was discovered that a correlation exists between the torsional barriers opposing the motions of methyl groups attached to peptide bonds and the degrees of their resonance delocalization. The Pratt group also has studied small molecules that act as “control elements” in biological systems, such as the neurotransmitters phenethylamine and p-methoxyphenethylamine, and the indole derivatives indole acetic acid, indole propionic acid, tryptophol, and tryptamine. Initially, the focus in these studies was on identifying the different conformers that appeared in the gas phase sample, since side-chain flexibility is an important issue in biological systems. Later, it became interesting to determine the relative stabilities of the different conformers and possible interconversions between them, with or without attached solvent molecules. These properties are controlled by intermolecular forces and are directly related to protein folding; hence, studies of these phenomena (by the Pratt group and many others) continue to this day. An intervening sabbatical at Middlebury College in Vermont and at Colby College in Maine in the winter of 2005 helped immensely to reconnect me to the collegiality of liberal arts colleges, of which I am so fond. Tom Shattuck and Whitney King remain good friends. This experience also gave me an opportunity to explore new venues for the natural science course that Peter Koehler, Pat Card, and I had developed at the University of Pittsburgh. A new tool was added to the high-resolution electronic spectroscopy “toolbox” with the development of a Stark cell for the molecular beam spectrometer by Tim Korter (with the help of Chris Butler, an undergraduate) and of the necessary software for modeling the behavior of the spectra in the presence of applied electric fields by David Borst and Cheol-Hwa Kang (with the help of Jon Hougen). Simulations of theoretical spectra showed that the experiment provides both the magnitude and the orientation of the permanent dipole vector in both electronic states. Aniline was the subject of the group’s first experimental study. This showed that the a-axis component of its permanent electric dipole moment in the excited S1 state increased by ∼150% compared with the corresponding value in the ground S0 state, a harbinger of significant intramolecular charge transfer. Later, similar studies of other benzene derivatives revealed that the total dipole moment of a molecule containing different functional groups is the vector sum of bond dipole moments, referred to the inertial frame. Jennifer Reese showed that the different conformers of 3-aminophenol had different dipole moments, making possible their spatial separation in high electric fields, as demonstrated in recent experiments by former undergraduate Jochen Kuepper at the FHI, Berlin. Korter and Kang used this new tool to measure the induced dipole moment in the hydrogen bonded complex indoleH2O. Then, Tri Nguyen showed that the permanent dipole moments of four tryptamine conformers in the gas phase were different, providing a new diagnostic of biomolecule structure and dynamics. More recently, Diane (Mitchell) Miller measured the permanent dipole moments of several “pushpull” molecules in their ground and electronically excited states, Jessica Thomas showed that the direction of the dipole is reversed on excitation of 1-phenylpyrrole to its S1 state, and A. J. Fleisher and his colleagues have given a convincing explanation for “anomalies” in the solvatochromic behaviors of different aminobenzoic acids on the basis of their 9333
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The Journal of Physical Chemistry A measured dipoles. Fleisher also has discovered evidence for “flickering” dipoles in the Stark-split high-resolution spectra of 2HNH2O. This is an exciting result, since large jumps in the angular orientations of water molecules produce time-varying electric fields that are thought to be important in bulk water, as well as in biological systems. A new focus of work of this sort will be on the changes in electron distribution that accompany complex formation, which may provide a deeper understanding of “induced fit” in prereactive complexes of importance in chemistry and biology. Rotations are slow compared with most other molecular motions. With the resolution that is currently available, the CW laser/molecular beam experiment is sensitive to motions that occur on time scales of 50 ns or less, leading to many dynamical applications of this technique. Already mentioned are the findings relating to internal rotation of methyl groups, as the measured differences in the rotational constants of different torsional levels are exquisitely sensitive to barrier heights. David Borst showed in his elegant study of the spectrum of toluene that, below the barrier, the attached methyl group tilts, and the ring bond lengths change with increasing displacements along the torsional coordinate. Above the barrier, the precessional motion of the CH3 group is quenched, but larger ring distortions are observed, providing a facile route to IVR. Conformer-selective IVR was observed in the spectra of other alkylbenzenes. Line broadening and perturbations were observed in postdoc Wayne Sinclair’s spectra of anilineAr, revealing in the frequency domain the important role of intra-intermolecular mode mixing in vibrational predissociation. Mode mixing has been detected in other types of large amplitude motions, most notably by Leo Alvarez in his experiments on 9,10-dihydrophenanthrene, substantially reducing the barrier to planarity. Diane Miller observed tunneling splittings in her study of 9-fluorenemethanol that made possible the determination of a π-hydrogen bond strength involving the attached CH2OH group and an aromatic ring. Tunneling splittings also were detected in the spectrum of the 2PY/2HP dimer. Deuterium substitution experiments by Rob Roscioli and instanton calculations by Willem Siebrand and co-workers showed that these splittings are caused by a concerted (ground-state) double proton transfer reaction along the OH 3 3 3 O and N 3 3 3 HN hydrogen bonds that hold the dimer together, substitution of the weaker and longer N 3 3 3 HN bond having the larger effect. Even when splittings are not observed, though, one can still deduce dynamical information from the spectra. Particularly interesting to the Pratt group are systems known to undergo proton or charge transfer reactions initiated by light, especially since at least two potential energy surfaces, the surfaces “before” and “after” the reaction has occurred, are often invoked to explain the observed dynamics. Thus, there are “locally excited” states, followed by “proton (or charge)-transfer” states. Although such descriptions are sometimes useful, they may not tell the full story. Recall the need for “mixed” states in the description of the process known as intersystem crossing. So the changes in the rotational constants of an acidbase complex such as 1/2HNA that were observed by Plusquellic and Humphrey on excitation by light have been interpreted by us as representing motion along the proton transfer coordinate. Only one (excited state) surface is needed for this description, not two. The degree of charge transfer in such states has been directly measured by Fleisher and his collaborators from their dipole moments. Similarly, Alexei Nikolaev measured the fully resolved electronic spectrum
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of DMABN and interpreted the observed changes in rotational constants in terms of motion along the TICT coordinate, since the molecule was found to be significantly twisted in the S1 state. Vibronic coupling and access to the conical intersection in indole has been measured and interpreted in a recent collaboration involving Michael Schmitt from Duesseldorf. Justin Young, a present group member, has detected proton transfer in the excited state of a salicylic acid derivative (FSA) by comparing its permanent electric dipole moment with that of the ground state. Excited state proton transfer reactions also have been revealed by line-broadening in the spectra of the FSA dimer and of 2-(20 -pyridyl)pyrrole by Philip Morgan, in collaboration with Jacek Waluk. Many challenges remain in this field. The most significant of them is the need for higher spectral resolution; the increasing size of the molecules that are being explored, particularly those of biological importance, requires better resolution to observe the rotational structure that is crucial to advancing molecular understanding, both of their structures and of their motions along different dynamical coordinates. In most cases, this resolution is limited by the Doppler effect; typically, a few megahertz in a wellcollimated molecular beam crossed perpendicularly by an equally well collimated laser beam. Again, extremely fortuitously, a solution to this problem was found when chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy was developed by Brooks Pate at the University of Virginia, a technology that he has freely shared with many research groups around the world, including mine. Because of the change in carrier frequency, the Doppler width has now been reduced by at least 3 orders of magnitude, making possible new experiments on (at least the electronic ground states of) much larger molecules. Grad student Ryan Bird and postdoc Vanesa Vaquero are leading this effort in Pittsburgh, with a “mini” CP-FTMW machine built by Pate and his students from UVa, principally Justin Neill. The future of ME spectroscopy in the gas phase looks bright for generations to come. I have been blessed with many things in life: good health; the opportunity to engage in what have been largely curiosity-driven experiments supported by an extremely talented team of postdocs, students, and support personnel, which led to many unexpected and intriguing discoveries in the lab; the continuous support by the U.S. National Science Foundation of my research program for more than 30 years; interactions and collaborations with an outstanding group of scientists from all over the world; the trust of thousands of families who have given me and my Pitt colleagues the honor of helping their children grow in their classrooms and undergraduate laboratories; and, perhaps most importantly, the support of my family. While science has been my passion, I would be remiss if I did not acknowledge the key family relationships that have added joy and love to my life, especially with my Mom and Dad, whom I miss every day; my sisters Martha and Debbie; my daughter Susan, son-in-law Peter Arndt, and grandsons Jacob and Angus; my son Jon; and my wife Kathleen, with whom life in the past 20 years has been more wonderful than one could possibly imagine. Last but not least, I would like to express my appreciation to my good friends and long-time collaborators David Plusquellic, Brooks Pate, and Ken Jordan, and to the Editors of this Journal, for their support of this issue. David W. Pratt
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