Theoretical study of the influence of para- and meta-substituents on X-pyridineHF hydrogen bonding

The effects of O⁻, N (CH₃)₂, NH (CH₃), NH₂, C₂H₅, CH₃, OH, F, Cl, OF, Br, NO₂ and substituents in para- and meta-positions on X-pyridineHF hydrogen bond has been studied by HF, B3LYP and MP2 methods using 6-311++G(d,p) basis set. The relationship between hydrogen bond formation energy ΔE and electron donating (or withdrawing) of substituents has been investigated. In this respect, population analysis has been performed by atoms in molecules (AIM) and natural bond orbital (NBO) theories. The results of AIM and NBO analyses are in good agreement with calculated energy values. The relationship between Hammett coefficient and complexation energy has been established and the ρ constant has been calculated for hydrogen bonding. There is a relationship between σ and ΔE with a correlation coefficient equal to 0.94.     

H NMR Dynamic study of thermal Z/E isomerization of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives: Barriers to rotation about CC bond

The rotational barriers between the configurational isomers of two structurally related push–pull 4-oxothiazolidines, differing in the number of exocyclic CC bonds, have been determined by dynamic ¹H NMR spectroscopy. The equilibrium mixture of (5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1a) in CDCl₃ at room temperature to 333 K consists of the E- and Z-isomers which are separated by an energy barrier ΔG^# 98.5 kJ/mol (at 298 K). The variable-temperature ¹H NMR data for the isomerization of ethyl (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)ethanoate (2b) in DMSO-d₆, possessing the two exocyclic CC bonds at the C(2)- and C(5)-positions, indicate that the rotational barrier ΔG^# separating the (2E,5Z)-2b and (2Z,5Z)-2b isomers is 100.2 kJ/mol (at 298 K). In a polar solvent-dependent equilibrium the major (2Z,5Z)-form (>90%) is stabilized by the intermolecular resonance-assisted hydrogen bonding and strong 1,5-type S · · · O interactions within the SCCCO entity. The ¹³C NMR Δδ_C(2)C(2′) values, ranging from 58 to 69 ppm in 1a–d and 49-58 ppm in 2a–d, correlate with the degree of the push-pull character of the exocyclic C(2)C(2′) bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2′) position in the following order: COPh COEt > CONHPh > CONHCH₂CH₂Ph. The decrease of the Δδ_C(2)C(2′) values in 2a–d has been discussed for the first time in terms of an estimation of the electron donor capacity of the S fragment on the polarization of the CC bonds.     

A theoretical study on NH bond dissociation enthalpies of oxo, thio and seleno carbamates and their N-protonated and N-deprotonated species

The effect of N-protonation and N-deprotonation on structure, NH bond dissociation enthalpies (BDEs) and stabilities of radicals formed on H-abstraction from nitrogen atom of carbamates and their thio- and seleno-analogs have been investigated. For those molecules where experimental results are available for comparison, the ROB3LYP/6-311++G(d,p)//B3LYP/6-31+G* theoretical level is in agreement within the estimated experimental uncertainty. The NH BDE of carbamates H₂NC(=X)YCH₃ [X = O; Y = O, S, Se] are higher but lower when X = S, Se and Y = O, S, Se in comparison to NH BDE of NH₃. DFT calculations indicate that the NH bond dissociation enthalpies are decreased by protonation and deprotonation at nitrogen atom; but the effect of deprotonation is rather smaller than the protonation. The variations are analyzed in terms of stabilities of molecules, their protonated and deprotonated species along with their respective radicals. The electron delocalization from nitrogen, X and Y atoms, electrostatic interactions, conjugative interactions and spin delocalization are the important factors affecting the stability. The spin delocalization and shift of radical center to chalcogen X (X = S, Se) are the main determinants for radical stability.     

Origin of rotational barriers of the N-N bond in hydrazine: NBO analysis

Hydrazine passes through two transition states, TS1 (phi = 0 degrees ) and TS2 (phi = 180 degrees ), in the course of internal rotation around its N-N bond. The origin of the corresponding rotational barriers in hydrazine has been extensively studied by experimental and theoretical methods. Here, we used natural bond orbital (NBO) analysis and energy decomposition of rotational barrier energy (DeltaE(barrier)) to understand the origin of the torsional potential energy profile of this molecule. DeltaE(barrier) was dissected into structural (DeltaE(struc)), steric exchange (DeltaE(steric)), and hyperconjugative (DeltaE(deloc)) energy contributions. In both transition states, the major barrier-forming contribution is DeltaE(deloc). The TS2 barrier is lowered by pyramidalization of nitrogen atoms through lowering DeltaE(struc), not by N-N bond lengthening through lowering DeltaE(steric). Higher pyramidality of nitrogen atoms of TS2 than that of TS1 explains well why the N-N bond of TS2 is longer than that of TS1. Finally, the steric repulsion between nitrogen lone pairs does not determine the rotational barrier; nuclear-nuclear Coulombic repulsion between outer H/H atoms in TS1 plays an important role in increasing DeltaE(struc). Taken together, we explain the reason for the different TS1 and TS2 barriers. We show that NBO analysis is a useful tool for understanding structures and potential energy surfaces of compounds containing the N-N bond.     

CN bond rotation and E–Z isomerism in some N-benzyl-N-methylcarbamoyl chlorides: A DFT study

The electronic and geometrical structures of thiosulfinic acids, RSSOH, 1, (1a, R = H; 1b, R = CH₃; 1c, R = t-C₄H₉; 1d, R = C₆H₅; 1e, R = F) and their anions have been investigated by the ab initio and density functional methods. The calculations show that the stability of the thiolo-tautomers of 1, RS(O)SH, and the thiono-tautomers, RS(S)OH, is almost the same in the gas phase, the energy of the thiolo-tautomers being slightly lower. The tautomers of 1 having the thiosulfone structure, RS(O)(S)H, were found to be the least stable. The thiosulfinate anions show ambident character with the negative charge dispersed over terminal O and S atoms and are stabilized by electronegative substituents.     

Rotation about the CN bond in thioamides: Influence of substituents on the potential function

Barriers to rotation about the CN bond in several unusual thioamides have been studied with 100 MHz ¹H-NMR spectroscopy. This rotation is still frozen at 170°C in p-chlorophenylglyoxylthiomorpholide (2) in DMSO-d₆. NMR spectra and the IR carbonyl frequencies show that 2b and 2c are the most important resonance forms, the latter making a significant contribution to the bonding in the transition state. The high rotational barrier is related to preferential stabilization of the ground state by the opposed orientation of two parallel dipoles (2b). In the sterically crowded molecules 4–(2′-hydroxythiobenzoyl)morpholine (3) and 1–(2′-hydroxythiobenzoyl)piperidine (4) rapid ring inversion between two chair forms of slightly different energy superimposed on the much slower rotation about the CN bond implies that only a mean value for the rotational barrier height can be obtained experimentally.     

New results in the ammonolysis of hexafluoro-cyclo-triphosphazene: Crystal structure of P3N3F5–NH–P3N3F4NH2

The reaction of hexafluoro-cyclo-triphosphazene P₃N₃F₆ with ammonia in acetonitrile has been studied. New compounds, (2-imino-2,4,4,6,6-pentafluoro-2λ⁵,4λ⁵,6λ⁵-cyclo-triphosphaza-1,3,5-trienyl)-2-amino-4,4,6,6-tetrafluoro-2λ⁵,4λ⁵,6λ⁵-cyclo-triphosphaza-1,3,5-triene, P₃N₃F₅–NH–P₃N₃F₄NH₂ (2) and cis and trans isomers of non-gem-2,4-diamino-2,4,6,6-tetrafluoro-2λ⁵,4λ⁵,6λ⁵-cyclo-triphosphaza-1,3,5-triene, P₃N₃F₄(NH₂)₂ (4, 5)_, were detected by GC/MS, and ³¹P NMR spectroscopy in reaction mixtures. X-ray diffraction analysis of P₃N₃F₅–NH–P₃N₃F₄NH₂ (2) revealed two conformational polymorphs, 2A and 2B, the latter being built up of two different conformers that were further denoted as 2B_a (the same as the single conformer in 2A) and 2B_b. The compound 2 was characterized by spectroscopic methods and its 2D potential energy surface (PES) was described by density functional theory computations depending on two dihedral angles. The calculated PES spans over 30 kJ/mol in energy including 8 local minima and all first and second order saddle points. The occurrence of the two experimentally observed conformers 2B_a and 2B_b seems to be governed by crystal packing effects.     

Ab initio and DFT conformational study on nitrosamine (H<Subscript>2</Subscript>N–N=O) and <Emphasis Type="Italic">N</Emphasis>-Nitrosodimethylamine [(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>N–N=O]

Abstract  

Ab initio and DFT calculations have been performed to characterize some ground state structures of the title molecules. Relative energies, rotational barriers, NBO charges, and dipole moments (μ) have been calculated and analyzed. It has been confirmed that only highly correlated methods (e.g., CCSD) are able to yield the non-planar structure as a minimum, for the H₂NNO molecule. On the other hand, all computational levels here employed are able to yield a planar C₂NNO frame for the (CH₃)₂NNO as a minimum. Important correlations between atomic charges and bond distances are discussed. Replacement of H by methyl group increases the rotational barrier and μ values by at least 3 kcal/mol and 0.4 D, respectively. The largest μ values are obtained for the structures in which the nitrogen lone pair is parallel to the NO group π system, and are consistent with a larger contribution of a dipolar resonance structure.     

Rotational isomerism inbis carbene MOL4 complexes: A theoretical study

LCAO-MO-SCF calculations are reported for the different stereoisomers of the Mo(CO)₄(CH₂)₂ and Mo(CO)₄[C(NH₂)₂]₂ systems. The substitution of the hydrogen atoms by the amino groups in the carbene ligands leads to an almost zero rotational barrier. Steric interactions are therefore expected to govern the barrier for diaminocarbene ligands which are more bulky than C(NH₂)₂. The rotational isomerism in thesebis carbene MoL₄ systems is also discussed in connection with the isolobal analogy between CH₂ and C₂H₄.     

A DFT study of Lp···π/halogen bond competition in complexes of perhalogenated alkenes with oxygen/nitrogen containing simple molecules

The possible noncovalent lone pair‐π/halogen bond (lp···π/HaB) complexes of perhalogenated unsaturated C₂Cl_nF_4?n (n = 0–4) molecules with four simple molecules containing oxygen or nitrogen as electron donor, formaldehyde (H₂CO), dimethyl ether (DME), NH₃, and trimethylamine (TMA), have been systematically examined at the M062X/aug‐cc‐pVTZ level. Natural bond orbital (NBO) analysis at the same level is used for understanding the electron density distributions of these complexes. The progressive introduction of Cl atom on C₂Cl_nF_4?n influences more on the lp···π complexes over the corresponding HaB ones. Within the scope of this study, gem‐C₂Cl₂F₂ is the best partner molecule for lp···π interaction with the simple molecules, coupled with the greatest interaction energy (IE) and second‐order orbital interaction [E(2) value], whereas C₂F₄ is the poorest one. The C₂Cl₃F·H₂CO and C₂Cl₄·H₂CO complexes exhibit reverse lp···π bonding, while the Z/E‐C₂Cl₂F₂·NH₃, C₂Cl₃F·NH₃ and C₂Cl₄·NH₃ complexes perform half‐lp···π bonding according to the NBO analysis. The lp···π interaction involving the oxygen/nitrogen and the π‐hole of C₂Cl_nF_4?n overwhelms the HaB involving the oxygen/nitrogen and the σ‐hole of the Cl atom. The electron‐donating methyl groups contribute significantly to the two competitive interactions, therefore, DME and TMA engage stronger in the partner molecules than H₂CO and NH₃. Our theoretical study would be useful for future experimental investigation on noncovalent complexes. © 2016 Wiley Periodicals, Inc.     

Interaction of N-(aryl)picolinamides with iridium. N–H and C–H bond activations

Reaction of N-(4-R-phenyl)picolinamide (R = OCH₃, CH₃, H, Cl and NO₂) with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords two yellow complexes (1-R and 2-R). The 1-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines, a chloride and a hydride. The 2-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines and two hydrides. Similar reaction of N-(naphthyl)picolinamide with [Ir(PPh₃)₃Cl] affords two organometallic complexes, 3 and 4. In complex 3 the amide ligand is coordinated to the metal center, via C–H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor, along with two triphenylphosphines and one chloride. Complex 4 is similar to complex 3, except a hydride is bonded to iridium instead of the chloride. Structures of the 1-OCH₃, 2-Cl and 4 complexes have been determined by X-ray crystallography. All the complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ir^III–Ir^IV oxidation within 0.50–1.16 V vs. SCE and a reduction of the coordinated amide ligand within −1.02 to −1.25 V vs. SCE.     

The catalytic effects of various third molecules on reaction channels of weakly bound complex CO2HF systems in the vapor phase

In this investigation, reaction channels of weakly bound complexes CO₂HF, CO₂HFH₂O, CO₂HFNH₃, CO₂HFCH₃OH, CO₂HFNH₂CH₃, CO₂HFNH(CH₃)₂ and CO₂HFN(CH₃)₃ systems were studied at the B3LYP/6-311++G(3df,2pd) level. The conformers of syn-fluoroformic acid or syn-fluoroformic acid plus a third molecule (H₂O, NH₃, CH₃OH, NH₂CH₃, NH(CH₃)₂ or N(CH₃)₃) were found to be more stable than the conformers of the related anti-fluoroformic acid or anti-fluoroformic acid plus a third molecule (H₂O, NH₃, CH₃OH, NH₂CH₃, NH(CH₃)₂ or N(CH₃)₃). However, the weakly bound complexes were found to be more stable than either the related syn- and anti- type fluoroformic acid or the acid plus third molecule (H₂O, NH₃, CH₃OH, NH₂CH₃, NH(CH₃)₂ or N(CH₃)₃) conformers. They decomposed into CO₂+HF, CO₂+H₃OF, CO₂+NH₄F, CO₂+(CH₃)OH₂F, CO₂+NH₃(CH₃)F, CO₂+NH₂(CH₃)₂F, or CO₂+NH(CH₃)₃F combined molecular systems. The weakly bound complexes have seven reaction channels, each of which includes weakly bound complexes and their related systems. Moreover, each reaction channel includes two transition state structures. The transition state between the weakly bound complex and anti-fluoroformic acid type structure (T₁₃) is significantly higher than that of internal rotation (T₂₃) between the syn- and anti-FCO₂H (or FCO₂HH₂O, FCO₂HNH₃, FCO₂HCH₃OH, FCO₂HNH₂CH₃, FCO₂HNH(CH₃)₂, or FCO₂HN(CH₃)₃) structures. However, adding the third molecule H₂O, NH₃, CH₃OH, NH₂CH₃, NH(CH₃)₂ or N(CH₃)₃ can significantly reduce the activation energy of T₁₃. The catalytic strengths of the third molecules are predicted to follow the order H₂O<NH₃<CH₃OH<.NH₂CH₃<NH(CH₃)₂<N(CH₃)₃.     

Theoretical investigations on R(O)nS-NO (n=0,1,2) systems

In the current article we report the ab initio study on the stability of S-Nitrosothiols (MeSNO, 1) and their oxidised derivatives (MeS(O)NO, 2) and (MeS(O)₂NO, 3). The bond length, bond order, rotational barrier and bond dissociation energy have been calculated and compared with that of sulfenamide (HS-NH₂) and its oxidised derivatives sulfinamide (H(O)S-NH₂) and sulfonamide (H(O)₂S-NH₂). The S-N bond dissociation energy in the oxidised state is very small compared to parent RSNO indicating the weakness of sigma bond. NBO analysis suggests that the negative hyperconjugative interactions are very strong in S-nitrosothiols and their oxidised derivatives, which weaken the sigma bond and facilitate the release of nitric oxide.     

Theoretical studies on electron delocalisation in selenourea

Ab initio and density functional calculations have been performed on the different possible structures of selenourea(su), urea(u) and thiourea(tu) to understand the extent of delocalisation in selenourea in comparison to urea and thiourea. Selenourea(su-1) withC ₂ symmetry has the minima on the potential energy surface at MP2(fu)/6-31+G* level. The C-N rotational barrier in selenourea is 8.69 kcal/mol, which is 0.29 and 0.11 kcal/mol more than that of urea and thiourea respectively at MP2(fu)/6-31+G* level. N-inversion barrier is 0.55 kcal/mol at MP2(fu)6-31+G* level. NBO analysis has been carried out to understand the nature of different interactions responsible for the electron delocalisation.     

Comparative evolved gas analytical and structural study on trans-diammine-bis(nitrito)-palladium(II) and platinum(II) by TG/DTA–MS, TG–FTIR, and single crystal X-ray diffraction

Detailed study on identification and thermal decomposition of solid title compounds 1 and 2 crystallized from the used aqueous ammonia solutions of Pd(NH₃)₂(NO₂)₂ and Pt(NH₃)₂(NO₂)₂, has been carried out. Beyond the composition of complexes 1 and 2, their trans square planar configuration have already been recognized by reference IR spectra and powder XRD patterns, nevertheless their exact molecular and crystal structure as of trans-Pd(NH₃)₂(NO₂)₂ (1, Pd-NN) and trans-Pt(NH₃)₂(NO₂)₂ (2, Pt-NN) has been determined by single crystal X-ray diffraction (R = 0.0515 and 0.0341), respectively. Despite their compositional and configuration analogy, they crystallize in different crystal systems and space groups. The crystals of 1 (Pd-NN) are triclinic (space group No. 2, P-1, a = 5.003(1) Å, b = 5.419(1) Å, c = 6.317(1) Å, α = 91.34(2)°, β = 111.890(10)°, γ = 100.380(10)°), while those of 2 (Pt-NN) are monoclinic (space group No. 5, C2, a = 7.4235(16) Å, b = 9.130(2) Å, c = 4.4847(10) Å, β = 99.405(7)°).The pyrolytic processes of 1 and 2 (which might be sensitive to shock and heat) have been followed by simultaneous thermogravimetric and differential thermal analysis (TG/DTA), while the evolved gaseous species have been traced in situ by online coupled TG/DTA–EGA–MS and TG–EGA–FTIR instruments in He and air. Pd and Pt powders, forming as final solid products in single step, are captured and checked by TG and XRD. Whilst the unified evolved gas analyses report evolution of N₂, H₂O, NH₃, N₂O, NO, and NO₂ gases as gaseous product components in the exothermic decomposition of both trans-Pd(NH₃)₂(NO₂)₂ (1) and trans-Pt(NH₃)₂(NO₂)₂ (2) starting from ca. 230 and 220 °C, in sealed crucibles with a pinhole on the top, respectively.     

Numerical Investigation of a Parallel-Plate Atmospheric-Pressure Nitrogen/Ammonia Dielectric Barrier Discharge

In this paper, a planar atmospheric-pressure dielectric barrier discharge (AP-DBD) of nitrogen mixed with ammonia (0?C2?%) is simulated using one-dimensional self-consistent fluid modeling with cell-centered finite-volume method. This AP-DBD is driven by a 30?kHz power source with distorted sinusoidal voltages. The simulated discharge current densities are found to be in good agreement with the experiment data in both phase and magnitude. The simulated results show that the discharges of N₂ mixed with NH₃ (0?C2?%) are all typical Townsend-like discharges because the ions always outnumber the electrons very much which leads to no quasi-neutral region in the gap throughout the cycle. N₂ ⁺ and N₄ ⁺ are found to be the most abundant charged species during and after the breakdown process, respectively, like a pure nitrogen DBD. NH₄ ⁺ increases rapidly initially with increasing addition of NH₃ and levels off eventually. In addition, N is the most dominant neutral species, except the background species, N₂ and NH₃, and NH₂ and H are the second dominant species, which increase with increasing added NH₃. The existence of abundant NH₂ plays an important role in those applications which require functional group incorporation.     

Efficient synthesis of new N-alkyl-d-ribono-1,5-lactams from d-ribono-1,4-lactone

d-Ribono-1,4-lactone was treated with ethylamine in DMF to afford N-ethyl-d-ribonamide 9a in quantitative yield. Bromination of amide 9a by the system SOBr₂ in DMF or PPh₃/CBr₄ in pyridine led, after acetylation, to epoxide 7. However, treatment of amide 9a with acetyl bromide in dioxane followed by acetylation gave 2,3,4-tri-O-acetyl-5-bromo-5-deoxyl-N-ethyl-d-ribonamide 10a. Methanolysis of 10a, with sodium methoxide, afforded the N-ethyl-d-ribonolactam 11a in 51% overall yields. Using this method, N-butyl, N-hexyl, N-dodecyl, and N-benzyl-d-ribonolactams 11b-e were obtained in good yields (48-53%).     

The structure of p-chlorophenol and barrier to internal OH rotation in the S1-state

The structure and barrier to internal rotation of 4-chlorophenol in the ground state and the electronically excited S₁-state has been examined by rotationally resolved laser induced fluorescence spectroscopy of 4-³⁵Cl-phenol, 4-³⁷Cl-phenol, 4-³⁵Cl-phenol-d₁, and 4-³⁷Cl-phenol-d₁. The overlapping spectra have been assigned simultaneously using a genetic algorithm approach. The rotationally resolved spectrum of the electronic origin of 4-chlorophenol is comprised of two subbands, which are split by 60 MHz due to the internal rotation of the hydroxy group. The torsional barrier in the electronically excited state could be estimated to be 1400 cm⁻¹, only about 250 cm⁻¹ higher than in the ground state. The CCl bond lengths decreases by approximately 6 pm upon electronic excitation and the aromatic ring is distorted quinoidally.     

Ab initio study on the mechanism of reaction HNCO+NH2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH₂+ HNCO?NH₃+NCO (1) and NH₂+HNCO?N₂H₃+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300–2700 K, transition theory rate constant for reaction (1) is 1.68×10¹¹–3.29×10¹¹ mL·mol^-1·s^-1, which is close to the experimental one of 5.0×10¹¹mL·mol^-1·s^-1or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes,cis andtrans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (forcis-mode) and 147.43 kJ/mol (fortrans-mode), respectively, which is much higher than reaction (1). So reaction (1) is the main channel for the titled reaction.     

Molecular orbital theory of the hydrogen bond. 27. Substituent effects in water: 4-R-pyrimidine complexes

Ab initio SCF calculations with the STO -3G basis set have been performed to investigate substituent effects on the structures and stabilization energies of water:4-R-pyrimidine complexes, with R including CH₃, NH₂, OH, F, C₂H₃, CHO, and CN. Except for the cyclic water:4-aminopyrimidine complex hydrogen bonded at N₃, these complexes have open structures stabilized by a nearly linear hydrogen bond formed through a nitrogen lone pair of electrons. When hydrogen bonding occurs at N₃, the complexes may have planar or perpendicular conformations depending on the substituent, but when hydrogen bonding occurs at N₁, the perpendicular is generally slightly preferred, and there is essentially free rotation of the 4-R-pyrimidine. Primary substituent effects alter the electronic environment at the nitrogens, and tend to make N₃ a poorer site for hydrogen bonding than N₁, primarily because of a stronger π electron-withdrawing effect at N₃. However, the relative stabilities of complexes hydrogen bonded at N₁ and N₃ are also influenced by secondary substituent effects, which may be significant in stabilizing complexes bonded at N₃. Substitutent effects on the structures and stabilization energies of the water:4-R-pyrimidine complexes are similar to substitutent effects in water:2-R-pyridine and water:4-R-pyrimidine complexes are similar to substitutent effects in water:2-R-pyridine and water:4-R-pyridine complexes. Configuration interaction calculations indicate that although absorption of energy by the pyrimidine ring destabilizes the water:4-R-pyrimidine complexes, these may still remain bound in the excited n → π* state. This is in contrast to the fate of open water:2-R-pyridine and water:4-R-pyridine complexes, which dissociate in this state.