Theoretical study on the mechanochemical reactivity in Diels-Alder reactions.

Mechanochemical reactions sometimes give different yields from those under solvent conditions, and such mechanochemical reactivities depend on the reactions. This study theoretically elucidates what governs mechanochemical reactivities, taking the Diels-Alder reactions as an example. Applying mechanical force can be regarded as the deformation of molecules, and the deformation in an orthogonal direction to a reaction mode can lower the reaction barrier. Here, we introduce a dimensionless cubic force constant, a mechanochemical reaction constant. It tells us how easily the deformation can lower a reaction barrier and enables us to compare the mechanochemical reactivities of different reactions. The constants correlate positively with the yields of the mechanochemical Diels-Alder reactions.

Origin of Stereoselectivity in a Mechanochemical Reaction of Diphenylfulvene and Maleimide.

Mechanochemical reactions sometimes yield unexpected products or product ratios in comparison to conventional reaction conditions. In the present study, we theoretically reveal the origin of the mechanochemical selectivity by considering the Diels-Alder reaction of diphenylfulvene and maleimide as an example. The application of an external force is equivalent to the production of a structural deformation. Here, we show that a mechanical force applied in a direction orthogonal to the reaction mode can lower the activation barrier by varying the potential energy curvature in the transition state. In the case of the Diels-Alder reaction, the endo-type pathway was found to be more mechanochemically favorable than the exo-type pathway, which is consistent with the experimental observations.

Carbazole-Dendronized Luminescent Radicals.

A series of carbazole-dendronized tris(2,4,6-trichlorophenyl)methyl (TTM) radicals have been synthesized. The photophysical properties of dendronized radicals up to the fourth generation were compared systematically to understand how structure-property relationships evolve with generation. The photoluminescence quantum yield (PLQY) was found to increase with the increasing generation, and the fourth generation (G4TTM) in cyclohexane solution showed a PLQY as high as 63 % at a wavelength of 627 nm (in the deep-red region) from the doublet state. The dendron modification strategy also showed a blue-shift of the emission on increasing the generation number, and the photostability was also increased compared to the bare TTM radical.

An Open-shell, Luminescent, Two-Dimensional Coordination Polymer with a Honeycomb Lattice and Triangular Organic Radical.

The use of organic radicals as building blocks is an effective approach to the production of open-shell coordination polymers (CPs). Two-dimensional (2D) CPs with honeycomb spin-lattices have attracted attention because of the unique electronic structures and physical properties afforded by their structural topology. However, radical-based CPs with honeycomb spin-lattices tend to have low chemical stability or poor crystallinity, and thus novel systems with high crystallinity and persistence are in strong demand. In this study, a novel triangular organic radical possessing three pyridyl groups, tris(3,5-dichloro-4-pyridyl)methyl radical (trisPyM) was prepared. It exhibits luminescence, high photostability, and a coordination ability, allowing formation of defined and persistent 2D CPs. Optical measurements confirmed the luminescence of trisPyM both in solution and in the solid state, with emission wavelengths, λem, of 665 and 700 nm, respectively. trisPyM exhibits better chemical stability under photoirradiation than other luminescent radicals: the half-life of trisPyM in CH2Cl2 was 10 000 times that of the tris(2,4,6-trichlorophenyl)methyl radical (TTM), a conventional luminescent radical. Complexation between trisPyM and ZnII(hfac)2 yielded a single crystal of a 2D CP trisZn, possessing a honeycomb lattice with graphene-like spin topology. The coordination structure of trisZn is stable under evacuation at 60 °C. Moreover, trisZn exhibits luminescence at 79 K, with λem = 695 nm, and is a rare example of a luminescent material among 2D radical-based CPs. Our results indicate that trisPyM may be a promising building block in the construction of a new class of 2D honeycomb CPs with novel properties, including luminescence.

A theoretical investigation into the role of catalyst support and regioselectivity of molecular adsorption on a metal oxide surface: NO reduction on Cu/γ-alumina.

The role of catalyst support and regioselectivity of molecular adsorption on a metal oxide surface is investigated for NO reduction on a Cu/γ-alumina heterogeneous catalyst. For the solid surface, computational models of the γ-alumina surface are constructed based on the Step-by-Step Hydrogen Termination (SSHT) approach. Dangling bonds, which appear upon cutting the crystal structure of a model, are terminated stepwise with H atoms until the model has an appropriate energy gap. The obtained SSHT models reflect the realistic infrared (IR) and ultraviolet-visible (UV/Vis) spectra. Vibronic coupling density (VCD), as a reactivity index, is employed to elucidate the regioselectivity of Cu adsorption on γ-alumina and that of NO adsorption on Cu/γ-alumina in place of the frontier orbital theory that could not provide clear results. We discovered that the highly dispersed Cu atoms are loaded on Lewis-basic O atoms, which is known as the anchoring effect, located in the tetrahedral sites of the γ-alumina surface. The role of the γ-alumina support is to raise the frontier orbital of the Cu catalyst, which in turn gives rise to the electron back-donation from Cu/γ-alumina to NO. In addition, the penetration of the VCD distribution of Cu/γ-alumina into the γ-alumina support indicates that the excessive reaction energy dissipates into the support after NO adsorption and reduction. In other words, the support plays the role of a heat bath. The NO reduction on Cu/γ-alumina proceeds even in an oxidative atmosphere because the Cu-NO bond is strong compared to the Cu-O2 bond.

CCR9 expression defines tolerogenic plasmacytoid dendritic cells able to suppress acute graft-versus-host disease.

Dendritic cells (DCs) are 'professional' antigen-presenting cells that are key in the regulation of immune responses. Here we characterize a unique subset of tolerogenic DCs that expressed the chemokine receptor CCR9 and migrated to the CCR9 ligand CCL25, a chemokine linked to the homing of T cells and DCs to the gut. CCR9(+) DCs were of the plasmacytoid DC (pDC) lineage, had an immature phenotype and rapidly downregulated CCR9 in response to maturation-inducing pDC-restricted Toll-like receptor ligands. CCR9(+) pDCs were potent inducers of regulatory T cell function and suppressed antigen-specific immune responses both in vitro and in vivo, including inhibiting acute graft-versus-host disease induced by allogeneic CD4(+) donor T cells in irradiated recipients. Our results identify a highly immunosuppressive population of pDCs present in lymphoid tissues.

Large Sessile Serrated Polyps Can Be Safely and Effectively Removed by Endoscopic Mucosal Resection.

BACKGROUND & AIMS: As many as 50% of large sessile serrated adenomas/polyps (SSPs) are removed incompletely, which is significant because SSPs have been implicated in the development of interval cancers. It is unclear if endoscopic mucosal resection (EMR) is an optimal method for removal of SSPs. We assessed the efficacy and safety of removal of SSPs 10 mm and larger using a standardized inject-and-cut EMR technique. METHODS: We performed a retrospective analysis of colonoscopy data, collected over 7 years (2007-2013) at 2 centers, from 199 patients with proximal colon SSPs 10 mm and larger (251 polyps) removed by EMR by 4 endoscopists. The primary outcome measure was local recurrence. The secondary outcome measure was safety. RESULTS: At the index colonoscopy, patients had a median of 1 serrated lesion (range, 1-12) and 1 nonserrated neoplastic lesion (range, 0-15). The mean SSP size was 15.9 ± 5.3 mm; most were superficially elevated (84.5%) and located in the ascending colon (51%), and 3 SSPs (1.2%) had dysplasia. Surveillance colonoscopies were performed on 138 patients (69.3%) over a mean follow-up period of 25.5 ± 17.4 months. Of these patients, 5 had local recurrences (3.6%; 95% confidence interval, 0.5%-6.7%), detected after 17.8 ± 15.4 months, with a median size of 4 mm. No patients developed postprocedural bleeding, perforation, or advanced colon cancer, or had a death related to the index colorectal lesion during the study period. CONCLUSIONS: Inject-and-cut EMR is a safe and effective technique for the resection of SSPs. Less than 5% of patients have a local recurrence, which is usually small and can be treated endoscopically.

Use of the over-the-scope clip to treat massive bleeding at the transitional zone of the anal canal: a case series.

BACKGROUND AND AIMS: Endoscopic treatment of anorectal bleeding can be challenging. We report use of the endoscopic over-the-scope clip (OTSC) to treat massive bleeding from the transitional zone of the anorectum. The aim of this retrospective study was to assess the clinical outcome and efficacy of the OTSC and to describe the technique of its use in acute severe bleeding occurring at the transitional zone of the anorectum. METHODS: We treated 5 consecutive patients (age range, 36-79 years, 5 men) with severe bleeding from the transitional zone of the anorectum caused by hemorrhoid therapy, digital trauma, and a Dieulafoy lesion. We analyzed the efficacy, safety, and outcome of endoscopic treatment using the OTSC. RESULTS: Primary hemostasis was successfully achieved in all the patients using a single OTSC. The OTSC deployment technique was adapted from the endoscopic band ligation of hemorrhoids. There was no immediate or late rebleeding. We observed that there were no adverse events from OTSC placement in the anorectum. CONCLUSIONS: This case series provides evidence that OTSCs may be effective in controlling bleeding from the transitional zone of the anorectum. Although use of OTSCs for bleeding elsewhere in the GI tract has been described, this case series is the first to show its application in the transitional zone of the anorectum.

Real-time optical diagnosis for diminutive colorectal polyps using narrow-band imaging: the VALID randomised clinical trial.

BACKGROUND: Diminutive (≤ 5 mm) colorectal polyps are common, and overwhelmingly benign. Routinely, after polypectomy, they are examined pathologically to determine the surveillance intervals. Advances in equipment and techniques, such as narrow-band imaging (NBI) colonoscopy, now permit reliable real-time optical diagnosis. METHODS: We conducted a randomised single-masked study involving three institutions to determine whether optical diagnosis of diminutive colorectal polyps meets clinical practice standards and reduces the need for histopathology. We randomly assigned eligible patients undergoing routine high-definition colonoscopy to optical diagnosis using near focus versus standard view, using computer-generated block sequence. By validated criteria, we rendered an optical diagnosis and a confidence level (high vs low) for all polyps, using NBI. Our primary endpoint was the number of accurate high-confidence optical diagnoses compared with central blinded pathology in the two groups. We analysed data using intention to treat. FINDINGS: We enrolled 558 subjects, and randomly assigned 281 to near focus and 277 to standard view optical diagnosis. We detected 1309 predominantly diminutive (74.5%) and neoplastic (60.0%) polyps. Endoscopists were significantly more likely, OR 2.2 (95% CI 1.6 to 3.0, p<0.0001), to make a high-confidence optical diagnosis with near focus (85.1%) than standard (72.6%) view. High-confidence diagnoses had 96.4% and 92.0% negative predictive value, respectively. Of all polyps, 75.3% (95% CI71.3% to 78.9%) had a high-confidence accurate prediction using near focus, compared with 63.1% (95% CI 58.5% to 67.6%) using standard view. Optical versus histopathological diagnosis showed excellent agreement between the surveillance intervals, 93.5% in near focus and 92.2% in standard view. The median diagnosis time was 14 s. CONCLUSIONS: Real-time optical diagnosis using NBI colonoscopy may replace the pathology diagnosis for the majority of diminutive colorectal polyps. Using colonoscopy with near focus view increases the confidence level of the optical diagnosis. Optical diagnosis would be a paradigm shift in clinical practice of colonoscopy for colorectal cancer screening. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov Identifier: NCT01288833.

Endoscopists can sustain high performance for the optical diagnosis of colorectal polyps following standardized and continued training.

BACKGROUND AND STUDY AIMS: The learning curve for optical diagnosis of colorectal polyps with the narrow-band imaging (NBI) is unknown. To forego histological analysis of diminutive polyps diagnosed optically with high confidence, guidelines recommend ≥ 90 % negative predictive value (NPV) and concordance of ≥ 90 % for surveillance intervals predicted optically and histologically. We aimed to study the learning of optical diagnosis for colorectal polyps. PATIENTS AND METHODS: We studied five endoscopists as part of a randomized multisite trial comparing near-focus and standard-focus views for optical diagnosis. They trained using a computer-based module, followed by 10 real-time colonoscopies with pathology correlation. Endoscopists then optically diagnosed and resected all the polyps found during 558 consecutive colonoscopies, and diagnoses were compared with pathology. Endoscopists repeated the training module at the study midpoint. NPV and concordance of surveillance intervals for diminutive polyps diagnosed optically with high confidence were measured over time. RESULTS: Endoscopists showed high diagnostic performance, with a nonsignificant trend toward higher NPV in the second half of the study. For the 445 polyps in the standard-view arm, the NPV was 88.0 % (95 %CI 75.7 % - 95.5 %) in the first half and 95.8 % (88.3 % - 99.1 %) in the second; P = 0.7. Three endoscopists in the first half and four in the second achieved > 90 % NPV. Concordance of surveillance intervals was identical in the first and second halves at 98.1 % (95 %CI 93.3 % - 99.8 %). CONCLUSIONS: High NPV for the prediction of non-neoplasms with NBI was achieved and maintained in this group of endoscopists who participated in standardized and continued training. Both NPV and surveillance interval agreement indicated high performance in the optical diagnosis of colorectal polyps and exceeded thresholds.

Reactivity of endohedral metallofullerene la2@c80 in nucleophilic and electrophilic attacks: vibronic coupling density approach.

The regioselectivities of La2@C80 in thermal nucleophilic and electrophilic attacks were theoretically investigated using vibronic coupling density (VCD) analysis. Nucleophilic and electrophilic cycloadditions to La2@C80 were experimentally reported to yield [6,6] and [6,5] adducts, respectively, as major products. VCD analysis provided a clear explanation for these experimental results. For nucleophilic reactions, it was found that the reactive [6,6] bonds did not have a large lowest unoccupied molecular orbital (LUMO) density and Fukui function but a large potential derivative with respect to a reaction mode. The VCD illustrates the origin of the interaction between the electronic and vibrational states. On the other hand, conventional reactivity indices such as frontier orbital density take only the electronic state into account. The result suggested that the stabilization due to vibronic couplings plays an important role in the regioselectivity of nucleophilic cycloadditions. The VCD with respect to the effective mode could provide a picture of the functional groups, which are the double bonds of ethylene moieties. VCD analysis with respect to hypothetical localized modes enabled the quantitative prediction of regioselectivities.

Low-lying electronic states in bismuth trimer Bi3 as revealed by laser-induced NIR emission spectroscopy in solid Ne.

Laser-induced near-infrared (NIR) emission spectra of neutral bismuth timer, Bi3, embedded in solid neon matrixes at 3 K were recorded in a range 870-1670 nm. Using photoexcitation with low energy photons at 1064 nm, two emission band systems were newly identified by their origin bands at T0 = 6600 and 8470 cm⁻¹. Accordingly, spectral assignment for three NIR emission band systems reported recently was partly revised for the one with its origin band at T0 = 7755 cm⁻¹ and reconfirmed for the others at T0 = 9625 and 11,395 cm⁻¹. Energy splitting by spin-orbit coupling between the pair of electronic energy levels in the ground state of bismuth trimer, Bi3, both having a totally symmetric vibrational mode of frequency at ω(e)" = 150 cm⁻¹, was determined to be 1870 ± 1.5 cm⁻¹. Transitions from the pair of electronically excited states, locating at T0 = 8470 and 9625 cm⁻¹ above the ground state and separated by spin­orbit coupling of 1155 cm⁻¹, have relatively long decay constants of τ ∼0.2 and ∼0.1 ms, respectively.

Quantum yield in blue-emitting anthracene derivatives: vibronic coupling density and transition dipole moment density.

A theoretical design principle for enhancement of the quantum yield of light-emitting molecules is desired. For the establishment of the principle, we focused on the S1 states of blue-emitting anthracene derivatives: 2-methyl-9,10-di(2'-naphthyl)anthracene (MADN), 4,9,10-bis(3',5'-diphenylphenyl)anthracene (MAM), 9-(3',5'-diphenylphenyl)-10-(3'',5''-diphenylbiphenyl-4''-yl) anthracene (MAT), and 9,10-bis(3''',5'''-diphenylbiphenyl-4'-yl) anthracene (TAT) [Kim et al., J. Mater. Chem., 2008, 18, 3376]. The vibronic coupling constants and transition dipole moments were calculated and analyzed by using the concepts of vibronic coupling density (VCD) and transition dipole moment density (TDMD), respectively. It is found that the driving force of the internal conversions and vibrational relaxations originate mainly from the anthracenylene group. On the other hand, fluorescence enhancement results from the large torsional distortion of the side groups in the S1 state. The torsional distortion is caused by the diagonal vibronic coupling for the lowest-frequency mode in the Franck-Condon (FC) S1 state, which originates from a small portion of the electron density difference on the side groups. These findings lead to the following design principles for anthracene derivatives with a high quantum yield: (1) reduction in the electron density difference and overlap density between the S0 and S1 states in the anthracenylene group to suppress vibrational relaxation and radiationless transitions, respectively; (2) increase in the overlap density in the side group to enhance the fluorescence.

Enhanced solid-state phosphorescence of organoplatinum π-systems by ion-pairing assembly.

Anion binding and ion pairing of dipyrrolyldiketone PtII complexes as anion-responsive π-electronic molecules resulted in photophysical modulations, as observed in solid-state phosphorescence properties. Modifications to arylpyridine ligands in the PtII complexes significantly impacted the assembling behaviour and photophysical properties of anion-free and anion-binding (ion-pairing) forms. The PtII complexes, in the presence of guest anions and their countercations, formed various anion-binding modes and ion-pairing assembled structures depending on constituents and forms (solutions and crystals). The PtII complexes emitted strong phosphorescence in deoxygenated solutions but showed extremely weak phosphorescence in the solid state owing to self-association. In contrast, the solid-state ion-pairing assemblies with tetraalkylammonium cations exhibited enhanced phosphorescence owing to the formation of hydrogen-bonding 1D-chain PtII complexes dispersed by stacking with aliphatic cations. Theoretical studies revealed that the enhanced phosphorescence in the solid-state ion-pairing assembly was attributed to preventing the delocalisation of the electron wavefunction over PtII complexes.

Combinatorial characterization of metastable luminous silver cations.

Thermodynamically metastable glasses that can contain metastable species are important functional materials. X-ray absorption near-edge structure (XANES) spectroscopy is an effective technique for determining the valence states of cations, especially for the doping element in phosphors. Herein, we first confirm the valence change of silver cations from monovalent to trivalent in aluminophosphate glasses by X-ray irradiation using a combination of Ag L3-edge XANES, electron spin resonance, and simulated XANES spectra based on first-principles calculations. The absorption edge of the experimental and simulated XANES spectra demonstrate the spectral features of Ag(III), confirming that AgO exists as Ag(I)Ag(III)O2. A part of Ag(I) changes to Ag(III) by X-ray irradiation, and the generation of Ag(III) is saturated after high irradiation doses, in good agreement with conventional radiophotoluminescence (RPL) behaviour. The structural modelling based on a combination of quantum beam analysis suggests that the local coordination of Ag cations is similar to that of Ag(III), which is confirmed by density functional theory calculations. This demonstration of Ag(III) in glass overturns the conventional understanding of the RPL mechanism of silver cations, redefining the science of silver-related materials.

Vibronic coupling density analysis for the chain-length dependence of reorganization energies in oligofluorenes: a comparative study with oligothiophenes.

The vibronic coupling constants and reorganization energies of oligofluorenes OF(n) (n = 1-6) are calculated for their cationic states (hole transport). Those of oligothiophenes OT(2n) (n = 1-6) are also calculated for comparison. The vibronic coupling constants of OF(n) are smaller than those of OT(2n), and decrease with increasing n. For the elucidation of the small vibronic couplings of the oligofluorenes, the calculated vibronic coupling constants are analyzed on the basis of the concept of vibronic coupling density. The vibronic coupling density of OF(n) becomes small in the middle of the chain with increasing n because of the reduction in the electron-density difference between the neutral and cationic states. It is found that orbital relaxation plays a crucial role in the distribution of the electron-density difference. From the fragment molecular orbital analyses, the large orbital relaxation in OF(n) is found to originate from the small transfer integral between the fragment molecular orbitals. These findings led to a design principle for a carrier-transporting oligomer/polymer with small vibronic couplings, or small reorganization energy, as follows: the orbital interaction between the monomers should be small from the view of vibronic couplings.

Localised surface plasmon resonance inducing cooperative Jahn-Teller effect for crystal phase-change in a nanocrystal.

The Jahn-Teller effect, a phase transition phenomenon involving the spontaneous breakdown of symmetry in molecules and crystals, causes important physical and chemical changes that affect various fields of science. In this study, we discovered that localised surface plasmon resonance (LSPR) induced the cooperative Jahn-Teller effect in covellite CuS nanocrystals (NCs), causing metastable displacive ion movements. Electron diffraction measurements under photo illumination, ultrafast time-resolved electron diffraction analyses, and theoretical calculations of semiconductive plasmonic CuS NCs showed that metastable displacive ion movements due to the LSPR-induced cooperative Jahn-Teller effect delayed the relaxation of LSPR in the microsecond region. Furthermore, the displacive ion movements caused photo-switching of the conductivity in CuS NC films at room temperature (22 °C), such as in transparent variable resistance infrared sensors. This study pushes the limits of plasmonics from tentative control of collective oscillation to metastable crystal structure manipulation.

Spontaneous-Symmetry-Breaking Charge Separation Induced by Pseudo-Jahn-Teller Distortion in Organic Photovoltaic Material.

The driving force of charge separation in the initial photovoltaic conversion process is theoretically investigated using ITIC, a nonfullerene acceptor material for organic photovoltaic devices. The density functional theory calculations show that the pseudo-Jahn-Teller (PJT) distortion of the S1 excimer state induces spontaneous symmetry-breaking charge separation between the identical ITIC molecules even without the asymmetry of the surrounding environment. The strong PJT effect arises from the vibronic coupling between the pseudodegenerate S1 and S2 excited states with different irreducible representations (irreps), i.e., Au for S1 and Ag for S2, via the asymmetric vibrational mode with the Au irrep. The vibrational mode responsible for the spontaneous polarization, which is opposite in one ITIC monomer and the other, is the intramolecular C-C stretching vibration between the core IT and terminal IC units. These results suggest that controlling the PJT effect can improve the charge separation efficiency of the initial photovoltaic conversion process.

Colonoscopy with clipping is useful in the diagnosis and treatment of diverticular bleeding.

BACKGROUND & AIMS: Diverticular bleeding is the most common cause of acute severe lower gastrointestinal bleeding (LGIB) in Western countries. Diagnostic and therapeutic approaches, including endoscopy, radiology, or surgery, have not been standardized. We investigated colonoscopy as a first-line modality to diagnose and manage patients with LGIB. METHODS: We performed a retrospective study of data collected from 2 tertiary Veterans hospitals of 64 patients (61 men, 76 ± 11 years) with acute severe diverticular bleeding, based on colonoscopy examination. We assessed primary hemostasis using endoscopic clipping for diverticular bleeding and described the bleeding stigmata. We measured early (<30 days) and late rebleeding, blood transfusion requirements, hospital stay and complications. RESULTS: Patients received 3.1 ± 3.0 and 0.9 ± 2.2 U of blood before and after colonoscopy, respectively. Twenty-four of the 64 patients (38%) had diverticular stigmata of recent hemorrhage; and 21 of these patients (88%) were treated successfully using endoscopic clips, without complication or early rebleeding. Hospital stays averaged 6.4 ± 5.6 days. Endoscopic clipping provided primary hemostasis in 9/12 patients (75%) with active diverticular bleeding. During 35 ± 18 months of follow-up, late recurrent diverticular bleeding occurred in 22% of the patients (14/64) after a mean time period of 22 months; 5 of the patients (21%) with stigmata of recent hemorrhage who received clip treatment had rebleeding at 43 months. Rebleeding was self-limited in 8 patients (57%), was clipped in 4 (29%), or was embolized in 2 (14%). CONCLUSIONS: Colonoscopy can be a safe first-line diagnostic and therapeutic approach for patients with severe LGIB. Endoscopic clipping provides hemostasis of active diverticular bleeding. Recurrent bleeding occurs in about 21% of patients who were treated with clips, at approximately 4 years; most bleeding is self-limited or can be retreated by endoscopic clipping.

Chemical reactivity in nucleophilic cycloaddition to C70: vibronic coupling density and vibronic coupling constants as reactivity indices.

The chemical reactivity in nucleophilic cycloaddition to C70 is investigated on the basis of vibronic (electron-vibration) coupling density and vibronic coupling constants. Because the e1″ LUMOs of C70 are doubly degenerate and delocalized throughout the molecule, it is difficult to predict the regioselectivity by frontier orbital theory. It is found that vibronic coupling density analysis for the effective mode as a reaction mode illustrates the idea of a functional group embedded in the reactive sites. Furthermore, the vibronic coupling constants for localized stretching vibrational modes enable us to estimate the quantitative reactivity. These calculated results agree well with the experimental findings. The principle of chemical reactivity proposed by Parr and Yang is modified as follows: the preferred direction is the one for which the initial vibronic coupling density for a reaction mode of the isolated reactant is a minimum.