Norrish type-I Research Articles

Versatile, Modular, and General Strategy for the Synthesis of α-Amino Carbonyls.

Modulating the basicity of alkylamines is a crucial factor in drug design. Consequently, alkylamines with a proximal amide, ester, or ketone have become privileged features in many pharmaceutical candidates. The impact of α-amino carbonyls has made the development of new methods for their preparation a continuous challenge in synthesis. Here, we describe a practical strategy that provides a modular and programmable synthesis of a wide range of α-amino carbonyls. The generality of this process is made possible by an extremely mild method to generate carbamoyl radicals, proceeding via a Lewis acid-visible-light-mediated Norrish type-I fragmentation of a tailored carboxamide reagent and intercepted through addition to in situ generated unbiased imines. Aside from the reaction's broad scope in each component, its capacity to draw on plentiful and diversely populated amine and carbonyl feedstocks is showcased through a two-dimensional array synthesis that is used to construct a library of novel, assay-ready, α-amino amides.

Conformers influence on UV-absorbance of avobenzone

Avobenzone is most commonly used FDA approved chemical UV-A filter in many sunscreen formulations. An important drawback of avobenzone is its photounstability and undergoes chemical transformation upon exposure to UV-radiation/sunlight. Eventhough single bond rotations are implicated in the radiation induced chemical modification of avobenzone, the conformers role of avobenzone on UV absorbance is yet to be addressed. Using the In silico platform working on Density Functional Theory (DFT), the communication evaluates the role of conformers on UV absorbance spectra of enol/keto forms of avobenzone. The results indicate that the conformers of enolic-form of avobenzone have broad range of absorbance from 273-389 nm and the keto-form has relatively narrow range of absorbance from 266-288 nm. The chelated conformers of enolic-avobenzone with intramolecular hydrogen bond are swingy in nature yet possessing strong bond contributing to its broad-spectrum UVA filtering activity. Among the conformers of keto-form of avobenzone, the planar conformer with the torsional angle 180° undergo facile Norrish Type-I cleavage. Mapping of the data with previous experimental results has allowed identifying 3D structures involved in the key steps of photoketonization process of avobenzone. The reported results may help to understand the molecular basis of enol-to-keto photochemical transformation of avobenzone and to develop photostabilizer to endure long-lasting photoprotection to avobenzone.

Photochemical Behavior of Some Estrone Aryl and Methyl Sulfonates in Solution: Preparative and Mechanistic Studies

Direct irradiation of estrone aryl and methyl sulfonates in different organic solvents under nitrogen atmosphere was investigated under steady-state conditions. The estrone derivatives reacted efficiently through the photo-Fries rearrangement reaction involving [1;3]-sulfonyl migration providing the ortho-sulfonyl estrone derivatives and estrone as the photoproducts. In addition, estrone and 2-arylsulfonyl estrone derivatives were epimerized involving a Norrish Type-I reaction. Chemical quenching and photosensitization experiments on the photoreaction have been also carried out to establish the photoreactive excited state. Likewise, the solvent effect and the nature of the sulfonyl group on the photoreactions have been also studied.

Effects of ring-strain on the ultrafast photochemistry of cyclic ketones.

Ring-strain in cyclic organic molecules is well-known to influence their chemical reactivity. Here, we examine the consequence of ring-strain for competing photochemical pathways that occur on picosecond timescales. The significance of Norrish Type-I photochemistry is explored for three cyclic ketones in cyclohexane solutions at ultraviolet (UV) excitation wavelengths from 255–312 nm, corresponding to an π* ← n excitation to the lowest excited singlet state (S1). Ultrafast transient absorption spectroscopy with broadband UV/visible probe laser pulses reveals processes common to cyclobutanone, cyclopentanone and cyclohexanone, occurring on timescales of ≤1 ps, 7–9 ps and >500 ps. These kinetic components are respectively assigned to prompt cleavage of an α C–C bond in the internally excited S1-state molecules prepared by UV absorption, vibrational cooling of these hot-S1 molecules to energies below the barrier to C–C bond cleavage on the S1 state potential energy surface (with commensurate reductions in the energy-dependent α-cleavage rate), and slower loss of thermalized S1-state population. The thermalized S1-state molecules may competitively decay by activated reaction over the barrier to α C–C bond fission on the S1-state potential energy surface, internal conversion to the ground (S0) electronic state, or intersystem crossing to the lowest lying triplet state (T1) and subsequent C–C bond breaking. The α C–C bond fission barrier height in the S1 state is significantly reduced by the ring-strain in cyclobutanone, affecting the relative contributions of the three decay time components which depend systematically on the excitation energy above the S1-state energy barrier. Transient infra-red absorption spectra obtained after UV excitation identify ring-opened ketene photoproducts of cyclobutanone and their timescales for formation.

Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls

Norrish Type I (NTI) α-bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In nonsymmetric species two cleavages are possible, NTIa and NTIb, forming larger and smaller alkyl radicals, respectively. For a data set of 20 small, atmospherically relevant carbonyls we predict NTIa and NTIb thresholds on the S0, S1, and T1 electronic states. The calculated NTIa T1 thresholds give a mean absolute deviation (MAD) of 5.8 kJ/mol with respect to the available experimental thresholds of five carbonyls. In addition, the intrinsic barrier heights to dissociation on the S0, S1, and T1 electronic states are predicted. We find RI-B2GP-PLYP/def2-TZVP calculations on S0 and unrestricted RI-B2GP-PLYP/def2-TZVP calculations on T1 give MADs of 6.1 kJ/mol for S0 asymptotic energies and 6.3 kJ/mol for S0 → T1 0-0 excitation energies, with respect to available experimental data. A composite method is used to determine S1 thresholds, with bt-STEOM-CCSD/cc-pVQZ calculation of vertical excitation energies and TD-RI-B3LYP/def2-TZVP calculations on S1, which achieves a MAD of 7.2 kJ/mol, with respect to experimental 0-0 excitation energies. Our calculations suggest, with the exception of bifunctional carbonyls and enones, NTI reactions on S1 are unlikely to be important at tropospherically relevant photolysis energies (<400 kJ/mol). In contrast, at these energies almost all possible NTI channels on T1 are open, and all barrierless S0 NTI dissociations are accessible. Our calculations allow a number of structural effects on both 0-0 excitation energies and intrinsic reaction barriers, on a given electronic state, to be elucidated and rationalized.

A Novel Pathway of Photoinduced Decomposition of Sphingolipids

A possibility of a novel photodestruction process has been shown for sphingolipids and their low-molecular analogues, including the stage of Norrish Type-I decomposition of excited molecules of the starting compounds followed by fragmentation of nitrogen-centered radicals via rupture of the C-C and O-H bonds. Realization of such transformation during photolysis of ceramide, sphingomyelin and galactocerebroside results in formation of 2-hexadecenal, which possess a wide spectrum of biological activity.

Comparison of Biological Activity Transistion Metal 12 Hydroxy oleate on Photodegradation of Plastics

The influence of low-molecular organic compounds such as Ferrous(Fe)12-hydroxyoleate and Nickel(Ni) 12-hydroxyloleate additives on photo transformation of polypropylene (PP) has been investigated. The tensile strength, infrared, optical properties and SEM has been used in these studies. The results indicate that additives applied accelerate and increase the efficiency of photodegradation in polypropylene but the hamper the photocrosslinking and formation of the carbonyl group following of the Norrish type-I in this polymer. In this case Ferrous 12-hydroxyl oleate showed greater activity as photo initiator comparing to Nickel 12-hydroxy oleate.

New insights into the photodynamics of acetylacetone: isomerization and fragmentation in low-temperature matrixes

UV and IR photoreactivities of acetylacetone isolated at 4.3 K in four matrixes (N(2), Ne, Ar, Xe), pure and doped with O(2) are investigated, using either tunable UV and IR optical parametric oscillators, or a broad band mercury lamp. Samples are probed by UV and FTIR spectroscopies: electronic and vibrational transitions are assigned and irradiation kinetics are analyzed. Contrary to what is observed in the gas phase, stereoisomerization is the main reaction observed: UV irradiation breaks the strong H-bond of the stable enolic form of acetylacetone, leading to the observation of non-chelated forms. Isomerization among the different non-chelated forms as well as back-isomerization to the chelated form are also observed under UV irradiation. Similar reactions and reaction rates are observed for the four matrixes, indicating that the inter-system crossing to the T(1) state involved in the isomerization process is very fast, probably due to efficient coupling with phonons, in contrast with gas phase where inter-system crossing is rate-limiting. When matrixes are doped with O(2), dissociation of the non-chelated forms under UV irradiation is observed and fragments, in particular CO, are formed in large amounts. Dissociation through a Norrish type-I reaction is probably one of the reaction channels occurring during electronic relaxation: dissociation is hindered by the surrounding cage in the case of pure matrixes while fragments immediately react with O(2) in the case of doped matrixes. The differences between gas phase and cold solid medium photodynamics of acetylacetone are discussed.

Radical reactions with double memory of chirality (2MOC) for the enantiospecific synthesis of adjacent stereogenic quaternary centers in solution: cleavage and bonding faster than radical rotation.

The solution photochemistry of bis(phenylpyrrolidinonyl)ketones (R,R)-1b and (S,S)-1b exhibited a remarkably high memory of chirality. Stereospecific decarbonylation to products (R,R)-3b and (S,S)-3b, respectively, occurred with an ee of ca. 80%. The reaction is thought to occur along the single state manifold by sequential Norrish type-I alpha-cleavage, decarbonylation, and radical-radical combination in a time scale that is comparable to that required for the radical intermediate to expose its other enantiotopic face by rotation about an axis perpendicular to that of the p orbital (ca. 3-7 ps). The absolute configuration of a key intermediate and that of ketone (R,R)-1b were determined by single-crystal X-ray diffraction and the ee values of the photochemical products with the help of chiral shift reagent (+)-Eu(tfc)(3) and chiral LC-MS/MS. On the basis of the ee and de values at 25 degrees C, it could be determined that ca. 70% of the bond forming events occur with double memory of chirality, ca. 21% occur after rotation of one radical to form the meso product (R,S)-3b, and only 9% occur after double rotation to form the opposite enantiomer. This report represents the first example of a doubly enantiospecific Norrish type-I and decarbonylation reaction in solution and illustrates potentially efficient ways to obtain compounds with adjacent stereogenic quaternary centers.

Contribution of Carbonyl Photochemistry to Aging of Atmospheric Secondary Organic Aerosol

The photodegradation of secondary organic aerosol (SOA) material by actinic UV radiation was investigated. SOA was generated via the dark reaction of ozone and d-limonene, collected onto quartz-fiber filters, and exposed to wavelength-tunable radiation. Photochemical production of CO was monitored in situ by infrared cavity ring-down spectroscopy. A number of additional gas-phase products of SOA photodegradation were observed by gas chromatography, including methane, ethene, acetaldehyde, acetone, methanol, and 1-butene. The absorption spectrum of SOA material collected onto CaF2 windows was measured and compared with the photolysis action spectrum for the release of CO, a marker for Norrish type-I photocleavage of carbonyls. Both spectra had a band at approximately 300 nm corresponding to the overlapping n --> pi* transitions in nonconjugated carbonyls. The effective extinction coefficient of freshly prepared SOA was estimated to be on the order of 15 L mol(-1) cm(-1) at 300 nm, implying one carbonyl group in every SOA constituent. The absorption by the SOA material slowly increased in the visible and near-UV during storage of SOA in open air in the dark, presumably as a result of condensation reactions that increased the degree of conjugation in the SOA constituents. These observations suggest that photolysis of carbonyl functional groups represents a significant sink for monoterpene SOA compounds in the troposphere, with an estimated lifetime of several hours over the continental United States.

In Vitro Phototoxic Properties of Triamcinolone 16,17-acetonide and Its Main Photoproducts¶

The phototoxicity of triamcinolone 16,17-acetonide has been estimated through a panel of in vitro tests. The main target involved in phototoxicity induced by triamcinolone appeared to be the cell membrane. Oxygen-independent photohemolysis was observed. A photochemical study in water and buffered solutions supported the conclusion that this is related to the action of radicals formed upon UV irradiation (in particular UV-B) by Norrish Type-I fragmentation of the C-20 ketone group. Peroxy radicals were formed in the presence of oxygen and were the active species in that case. Three photoproducts, isolated from the photodegradation of the drug, were submitted to the same toxicity tests. Two of them were proved to possess toxic or phototoxic properties on erythrocytes, primarily induced by UV-B light, and may participate in the photosensitizing activity of triamcinolone 16,17-acetonide. Our in vitro results suggest that the drug can elicit weak photosensitizing properties in vivo.

In vitro phototoxic properties of triamcinolone 16,17-acetonide and its main photoproducts.

The phototoxicity of triamcinolone 16,17-acetonide has been estimated through a panel of in vitro tests. The main target involved in phototoxicity induced by triamcinolone appeared to be the cell membrane. Oxygen-independent photohemolysis was observed. A photochemical study in water and buffered solutions supported the conclusion that this is related to the action of radicals formed upon UV irradiation (in particular UV-B) by Norrish Type-I fragmentation of the C-20 ketone group. Peroxy radicals were formed in the presence of oxygen and were the active species in that case. Three photoproducts, isolated from the photodegradation of the drug, were submitted to the same toxicity tests. Two of them were proved to possess toxic or phototoxic properties on erythrocytes, primarily induced by UV-B light, and may participate in the photosensitizing activity of triamcinolone 16,17-acetonide. Our in vitro results suggest that the drug can elicit weak photosensitizing properties in vivo.

Laser flash photolysis of [3,n]paracyclophan-2-ones. Direct observation and chemical behavior of 4,4'-(1,n-alkanediyl)bisbenzyl biradicals.

The 4,4'-(1,n-alkanediyl)bisbenzyl biradicals (2b-d) have been generated from the Norrish type-I reaction of [3,n]paracyclophan-2-ones (1b-d) giving the paracyclophanes 3b-d as the only reaction products. The behavior of biradicals 2b-d has been studied in detail and compared with the previously reported biradical 2a. The lifetimes increase as the chain length decreases and are affected by the solvent viscosity, thus showing the effect of the length of the chain on the conformations of the biradicals. Quenching with persistent radicals such as TEMPO resulted in length-dependent rate constants. Finally, the study of the magnetic field effects on the biradical lifetimes suggest that ISC control determines biradical lifetimes for long-chain systems.

Femtochemistry of Norrish Type-I Reactions: IV. Highly Excited Ketones—Experimental

Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S2 surface involving the C-C bond in a manner which is similar to that of ketones in the S1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.

Femtochemistry of Norrish type-I reactions: III. Highly excited ketones--theoretical.

Time-dependant density functional theory (TDDFT) and ab initio methods (CASSCF and CASMP2) are applied here for the investigation of the excited-state potential energy surfaces of ketones studied experimentally in the accompanying paper, number IV in the series. The aim is to provide a general and detailed physical picture of the Norrish type-I reaction from S0 and S1 potentials (papers I and II) and from higher-energy potentials (papers III and IV). Particular focus here is on reactions following excitation to the 3s, 3p, and 3d Rydberg state and to the (nz-->pi*) and (pi-->pi*) valence states. It is shown that the active orbitals in the CASSCF calculations can be chosen so that accurate results are obtained with a small active space. Dynamic corrections of the state-specific CASSCF energies at the multireference MP2 level do not improve the results for the Rydberg states but are significant for the valence states. The geometries of the Rydberg states are similar to the ground state; the S1 and other valence states are not. A common property of the valence states is the elongated CO bond and the pyramidalization of the carbonyl carbon atom. As a consequence, these valence states cross all Rydberg states along the CO stretching coordinate and provide an efficient pathway down to the 3s Rydberg states (S2) through a series of conical intersections (CIs). The nonadiabatic coupling vector of the CI between the (pi-->pi*) and the 3s Rydberg states guides energy channeling into the asymmetric CC-stretching mode. The energy demand for the CC bond breakage (Norrish type-I) on the S2 surface is lower than that of the CI leading to the S1 state. This CC bond breakage leads to a linear excited state acetyl radical (3s Rydberg). Crossing a small barrier the 3s acyl radical can access a CI leading either to a second CC bond breakage or to a hot ground-state acetyl radical. The barriers for the Norrish type-I reaction on the various excited-state surfaces can be rationalized within the framework of valence-bond theory. The dynamic picture of the Norrish type-I reactions is now clear: The excitation to high-energy states leads to the nonconcerted breakage of the alpha-CC bonds by an "effective downhill" potential in space involving the active excitation center CO, CC stretching, and CCO bending nuclear motions, but not, as usually thought, a direct repulsive potential along the CC bond. In our accompanying paper (part IV), it is shown that the results from the experimental investigations of Norrish type-I reactions on the femtosecond timescale are consistent with these theoretical results.

Femtochemistry of Norrish type-I reactions: IV. Highly excited ketones--experimental.

Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S2 surface involving the C-C bond in a manner which is similar to that of ketones in the S1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.

Cover Picture

The cover picture shows the products arising from the dissociation dynamics of cyclobutanone (CB). Unlike simpler linear ketones, CB predissociates anomalously under the Norrish type-I (α-cleavage) reaction mechanism. Investigation of partially deuterated CB isotopomers by femtosecond time-resolved mass spectrometry (shown in the inset), in combination with ab initio calculations, characterized the S0, S1(n,π), and T1(n,π*) potential energy surfaces. The passage through the conical intersection joining the S0 and S1 surfaces follow either gradient difference (x1) or a nonadiabatic coupling (x2) vectors: Following x2, either the hot parent molecule (left) or the acyl diradical (right) is produced; following x1 either the trimethylene and CO (middle) or hot cyclopropane and CO (not shown) are formed. Find out more in the articles by Zewail et al. on pages 273–293 and 294–309.

Femtochemistry of Norrish Type-I Reactions: I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S1 Surface

Femtochemistry of Norrish Type-I Reactions: I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S1 Surface

Femtochemistry of Norrish Type-I Reactions: I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S1 Surface

The dissociation dynamics of two acetone isotopomers ([D0 ]- and [D6 ]acetone) after 93 kcal mol(-1) (307 nm) excitation to the S1 (n,π*) state have been investigated using femtosecond pump-probe mass spectrometry. We found that the nuclear motions of the molecule on the S1 surface involve two time scales. The initial femtosecond motion corresponds to the dephasing of the wave packet out of the Franck-Condon region on the S1 surface. For longer times, the direct observation of the build-up of the acetyl radical confirms that the S1 α-cleavage dynamics of acetone is on the nanosecond time scale. Density functional theory and ab initio calculations have been carried out to characterize the potential energy surfaces for the S0 , S1 , and T1 states of acetone and six other related aliphatic ketones. For acetone, the S1 energy barrier along the single α-positioned carbon-carbon (α-CC) bond-dissociation coordinate (to reach the S0 /S1 conical intersection) was calculated to be 18 kcal mol(-1) (∼110 kcal mol(-1) above the S0 minimum) for the first step of the nonconcerted α-CC bond cleavage; the concerted path is energetically unfavorable, consistent with experiments. The S1 barrier heights for other aliphatic ketones were found to be substantially lower than that of acetone by methyl substitutions at the α-position. The α-CC bond dissociation energy barrier of acetone on the T1 surface was calculated to be only 5 kcal mol(-1) (∼90 kcal mol(-1) above the S0 minimum), which is substantially lower than the barrier on the S1 surface. Based on the calculations, the α-cleavage reaction mechanism of acetone occurring on the S0 , S1 , and T1 surfaces can be better understood via a simple physical picture within the framework of valence-bond theory. The theoretical calculations support the conclusion that the observed nanosecond-scale S1 dynamics of acetone below the barrier is governed by a rate-limiting S1 →T1 intersystem crossing process followed by α-cleavage on the T1 surface. However, at high energies, the α-cleavage can proceed by barrier crossing on the S1 surface, a situation which is demonstrated for cyclobutanone in the accompanying paper.

Femtochemistry of Norrish Type-I Reactions: II. The Anomalous Predissociation Dynamics of Cyclobutanone on the S1 Surface

The anomalous nonradiative dynamics for three cyclobutanone isotopomers ([D0 ]-, 3,3-[D2 ]-, and 2,2,4,4-[D4 ]cyclobutanone) have been investigated using femtosecond (fs) time-resolved mass spectrometry. We have found that the internal motions of the molecules in the S1 state above the dissociation threshold involve two time scales. The fast motion has a time constant of <50 fs, while the slow motion has a time constant of 5.0±1.0, 9.0±1.5, and 6.8±1.0 ps for the [D0 ], [D2 ], and [D4 ] species, respectively. Density functional theory and ab initio calculations have been performed to characterize the potential energy surfaces for the S0 , S1 (n,π*), and T1 (n,π*) states. The dynamic picture for bond breakage is the following: The fast motion represents the rapid dephasing of the initial wave packet out of the Franck-Condon region, whereas the slow motion reflects the α-cleavage dynamics of the Norrish type-I reaction. The redistribution of the internal energy from the initially activated out-of-plane bending modes into the in-plane ring-opening reaction coordinate defines the time scale for intramolecular vibrational energy redistribution (IVR), and the observed picosecond-scale (ps) decay gives the rate of IVR/bond cleavage across the barrier. The observed prominent isotope effect for both [D2 ] and [D4 ] isotopomers imply the significance of the ring-puckering and the CO out-of-plane wagging motions to the S1 α-cleavage dynamics. The ethylene and ketene (C2 products)-as well as CO and cyclopropane (C3 products)-product ratios can be understood by the involvement of an S0 /S1 conical intersection revealed in our calculations. This proposed dynamic picture for the photochemistry of cyclobutanone on the S1 surface can account not only for the abnormally sharp decrease in fluorescence quantum yield and lifetime but also for the dramatic change in the C3 :C2 product ratio as a function of increasing excitation energy, as reported by Lee and co-workers (J. C. Hemminger, E. K. C. Lee, J. Chem. Phys. 1972, 56, 5284-5295; K. Y. Tang, E. K. C. Lee, J. Phys. Chem. 1976, 80, 1833-1836).