Intermediate Trans Research Articles

Hydrogen-Atom-Assisted Uphill Isomerization of N-Methylformamide in Darkness.

N-Methylformamide, HC(O)NH(CH3), is the smallest amide detected in the interstellar medium that can exist as cis and trans isomers. We performed reactions of H atoms with trans-NMF in solid para-hydrogen at 3.3 K and found that the cis-NMF isomer, which has higher energy, increased continuously in darkness, demonstrating a previously overlooked and seemingly unlikely isomerization of prebiotic molecules through H-atom tunneling reactions in the absence of light. Infrared spectra of radical intermediates trans-•C(O)NH(CH3) and trans-HC(O)NH(•CH2) were identified. Further H addition and H abstraction enhanced the formation of CH3NCO, HNCO, and CH2NH in the H-rich experiments. These results indicate that, unlike the dual cycle of H-abstraction and H-addition channels chemically linking formamide and HNCO, the H addition to CH3NCO produced only cis-radicals that led to cis-NMF. Furthermore, H-atom-induced fragmentation by breaking the C-C bond provides links between NMF and HCNO/CH2NH. These endothermic isomerization/decomposition reactions become possible through the coupling with H + H → H2.

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity.

The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2-di-t-butyl-phosphinophenyl)phosphine (tBuPHPP), has been synthesized. When complexed to iridium, it adopts a meridional ("pincer") configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based-pincer (tBuPPP) complex. Stoichiometric reactions of the (tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded; one coordination site cis to the phosphido is even more crowded; and the remaining site is particularly open. The (tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous (RPCP)Ir catalysts. Accordingly, DFT calculations predict that (tBuPPP)Ir and (tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (RPCP)Ir proceeds through an intermediate trans-(PCP)IrH2(alkene), (tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH2(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (tBuPPP)Ir and (tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Thus, the resting state of the (tBuPPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

Photochemistry of Heteroleptic 1,4,5,8-Tetraazaphenanthrene- and Bi-1,2,3-triazolyl-Containing Ruthenium(II) Complexes.

Diimine metal complexes have significant relevance in the development of photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) applications. In particular, complexes of the TAP ligand (1,4,5,8-tetraazaphenanthrene) are known to lead to photoinduced oxidation of DNA, while TAP- and triazole-based complexes are also known to undergo photochemical ligand release processes relevant to PACT. The photophysical and photochemical properties of heteroleptic complexes [Ru(TAP)n(btz)3-n]2+ (btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 1 (1), 2 (2)) have been explored. Upon irradiation in acetonitrile, 1 displays analogous photochemistry to that previously observed for [Ru(bpy)(btz)2]2+ (bpy = 2,2'-bipyridyl) and generates trans-[Ru(TAP)(btz)(NCMe)2]2+ (5), which has been crystallographically characterized, with the observation of the ligand-loss intermediate trans-[Ru(TAP)(κ2-btz)(κ1-btz)(NCMe)]2+ (4). Complex 2 displays more complicated photochemical behavior with not only preferential photorelease of btz to form cis-[Ru(TAP)2(NCMe)2]2+ (6) but also competitive photorelease of TAP to form 5. Free TAP is then taken up by 6 to form [Ru(TAP)3]2+ (3) with the proportion of 5 and 3 observed to progressively increase during prolonged photolysis. Data suggest a complex set of reversible photochemical ligand scrambling processes in which 2 and 3 are interconverted. Computational DFT calculations have enabled optimization of geometries of the pro-trans 3MCcis states with repelled btz or TAP ligands crucial for the formation of 5 from 1 and 2, respectively, lending weight to recent evidence that such 3MCcis states play an important mechanistic role in the rich photoreactivity of Ru(II) diimine complexes.

Nuances in Fundamental Suzuki–Miyaura Cross-Couplings Employing [Pd(PPh3)4]: Poor Reactivity of Aryl Iodides at Lower Temperatures

We have explored fundamental Pd-catalyzed Csp2–Csp2 Suzuki–Miyaura cross-couplings of aryl iodides (Ar–I) employing “classical” Pd/PPh3 catalyst systems. Surprisingly, we observed particularly inefficient couplings of these ostensibly reactive electrophiles in a range of conventional solvent mixtures at lower temperatures (∼50 °C), which was in stark contrast to analogous reactions featuring the equivalent aryl bromides. This feature of well-established Pd/PPh3-mediated Suzuki–Miyaura reactions has received scant attention in the literature. Most significantly, our studies suggest that the inefficient coupling of aryl iodides at lower temperatures derives from the unexpectedly poor turnover of the key on-cycle intermediate trans-[Pd(PPh3)2(Ar)(I)] (or related PdII–I species) in the presence of PPh3.

Ruthenium Carbene Complexes Analogous to Grubbs-I Catalysts Featuring Germylenes as Ancillary Ligands

Reactions of the first-generation Grubbs’ catalyst trans-[RuCl2(CHPh)(PCy3)2] (1) with the amidinatogermylenes Ge(tBu2bzam)R (R = tBu (L1), CH2SiMe3 (L2); tBu2bzam = N,N′-bis(tertbutyl)benzamidinate) have allowed the isolation and full characterization of the first specimens of Grubbs-type carbene complexes featuring heavier tetrylenes as ancillary ligands, namely, the disubstituted derivatives trans-[RuCl2(CHPh)(L1)2] (3) and cis-[RuCl2(CHPh)(L2)2] (7), which curiously differ in the arrangement of their germylene ligands. DFT calculations have revealed that the different volumes of L1 and L2 (the former is larger than the latter) are responsible for the different stereochemistry of 3 and 7. NMR-monitoring of the reaction solutions has allowed the observation of the monosubstituted intermediates trans-[RuCl2(CHPh)(L)(PCy3)] (L = L1 (2), L2 (5)) and their evolution to either the disubstituted final product (for L1) trans-[RuCl2(CHPh)(L1)2] (3) or the short-lived disubstituted intermediate (for L2) trans-[RuCl2(CHPh)(L2)2] (6). Complex 7 arises from a trans-to-cis isomerization of intermediate 6. As olefin metathesis catalysts, both 3 and 7 promoted the ring-closing metathesis of diethyl 2,2-diallylmalonate and the ring-opening metathesis polymerization of norbornene, but their catalytic activity decreased with the reaction time, indicating catalyst decomposition.

Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)2(η2-O2)

Reaction of [Pd(IPr)2] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and O2 leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr)2(η2-O2)]. At temperatures ≳ -40 °C, cis-[Pd(IPr)2(η2-O2)] adds a second O2 to form trans-[Pd(IPr)2(η1-O2)2]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex. In addition to reaction with O2, cis-[Pd(IPr)2(η2-O2)] reacts at low temperature with H2O in methanol/ether solution to form trans-[Pd(IPr)2(OH)(OOH)]. The crystal structure of trans-[Pd(IPr)2(OOH)(OH)] is reported. Neither reaction with O2 nor reaction with H2O occurs under comparable conditions for cis-[Pd(IMes)2(η2-O2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr)2(η2-O2)] is attributed to the enthalpy of binding of O2 to [Pd(IPr)2] (-14.5 ± 1.0 kcal/mol) that is approximately one-half that of [Pd(IMes)2] (-27.9 ± 1.5 kcal/mol). Computational studies identify the cause as interligand repulsion forcing a wider C-Pd-C angle and tilting of the NHC plane in cis-[Pd(IPr)2(η2-O2)]. Arene-arene interactions are more favorable and serve to further stabilize cis-[Pd(IMes)2(η2-O2)]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O2 binding. A complete reaction diagram is constructed for formation of trans-[Pd(IPr)2(η1-O2)2] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd(IMes)2(η1-O2)2] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate trans-[Pd(NHC)2(η1-O2)] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O2 is constructed. A range of low-energy pathways at different angles of approach are present and blur the distinction between pure "side-on" or "end-on" trajectories for oxygen binding.

Dynamic Inversion of Stereoselective Phosphate Binding to a Bisurea Receptor Controlled by Light and Heat

AbstractA chiral bisurea anion receptor, derived from a first‐generation molecular motor, can undergo photochemical and thermal isomerization operating as a reconfigurable system. The two possible cis configurations in the isomerization cycle are opposite in helicity, as is shown by CD spectroscopy. 1H NMR titrations demonstrate that the P and M helical cis isomers hold opposite enantioselectivity in the binding of binol phosphate, while anion complexation by the intermediate trans isomer is not selective. The difference in the binding affinity of the enantiomers was rationalized by DFT calculations, revealing very distinct binding modes. Thus, the enantiopreferred substrate binding in this receptor can be inverted in a dynamic fashion using light and heat.

Dynamic Inversion of Stereoselective Phosphate Binding to a Bisurea Receptor Controlled by Light and Heat.

A chiral bisurea anion receptor, derived from a first-generation molecular motor, can undergo photochemical and thermal isomerization operating as a reconfigurable system. The two possible cis configurations in the isomerization cycle are opposite in helicity, as is shown by CD spectroscopy. (1)H NMR titrations demonstrate that the P and M helical cis isomers hold opposite enantioselectivity in the binding of binol phosphate, while anion complexation by the intermediate trans isomer is not selective. The difference in the binding affinity of the enantiomers was rationalized by DFT calculations, revealing very distinct binding modes. Thus, the enantiopreferred substrate binding in this receptor can be inverted in a dynamic fashion using light and heat.

Practical Asymmetric Hydrogenation-Based Synthesis of a Class-Selective Histone Deacetylase Inhibitor

Two syntheses of the class-selective histone deacetylase inhibitor 1 are reported. In the first, eight-step entailing synthesis, the key transformations were a highly efficient [3 + 2] dipolar cycloaddition affording trans-rac-5 and its resolution. In the second, asymmetric approach, the key steps were a highly selective asymmetric hydrogenation to produce the cis-(S,S)-3,4-disubstituted pyrrolidine 18 followed by an amide formation with simultaneous chiral inversion of the carboxy stereocenter to generate the key intermediate trans-(R,S)-3,4-disubstituted pyrrolidine 19. The overall yield increased from ∼6% for the resolution approach to ∼26% for the enantioselective approach.

An expedient one-pot sequential three-component reaction for the stereoselective synthesis of functionalized spiro-sulfamidate imine fused δ-lactone scaffold

A convenient one-pot three-component transformation between 4-aryl-5H-1,2,3-oxathiazole-2,2-dioxides, trans-β-aryl/alkyl-substituted acroleins and paraformaldehyde in CH2Cl2 at room temperature in the presence of commercially available l-proline and DBU as the organocatalysts has been successfully realized for the first time via a Michael-condensation-hemiacetalization sequence process. The resulting functionalized spiro-sulfamidate imine fused δ-lactone scaffolds were obtained via in situ oxidation of spiro-δ-lactols by PCC in good to excellent overall yields and moderate to good diastereomeric ratio (up to ⩽5:1 dr). Moreover, a synthetically useful intermediate trans–cis-β-amino-α-azido compound has been successfully achieved through our procedure.

Three roles for the fluoride ion in palladium-catalyzed Hiyama reactions: transmetalation of [ArPdFL₂] by Ar'Si(OR)₃.

From the kinetic data on the transmetalation/reductive elimination in fluoride-promoted Hiyama reactions, obtained using electrochemical techniques, it has been established that fluoride ions play three roles. F(-) reacts with trans-[ArPdBrL2] (L=PPh3) to form trans-[ArPdFL2], which reacts with Ar'Si(OMe)3 in the rate-determining transmetalation, whereas trans-[ArPdBrL2] does not react with Ar'Si(OMe)3. F(-) reacts with Ar'Si(OMe)3 to deliver the unreactive silicate Ar'SiF(OMe)3(-), thus leading to two antagonistic kinetic effects. In addition, F(-) catalyzes the reductive elimination from intermediate trans-[ArPdAr'L2].

Three Roles for the Fluoride Ion in Palladium‐Catalyzed Hiyama Reactions: Transmetalation of [ArPdFL2] by Ar′Si(OR)3

AbstractFrom the kinetic data on the transmetalation/reductive elimination in fluoride‐promoted Hiyama reactions, obtained using electrochemical techniques, it has been established that fluoride ions play three roles. F− reacts with trans‐[ArPdBrL2] (L=PPh3) to form trans‐[ArPdFL2], which reacts with Ar′Si(OMe)3 in the rate‐determining transmetalation, whereas trans‐[ArPdBrL2] does not react with Ar′Si(OMe)3. F− reacts with Ar′Si(OMe)3 to deliver the unreactive silicate Ar′SiF(OMe)3−, thus leading to two antagonistic kinetic effects. In addition, F− catalyzes the reductive elimination from intermediate trans‐[ArPdAr′L2].

Mechanism of Palladium‐Catalyzed Suzuki–Miyaura Reactions: Multiple and Antagonistic Roles of Anionic “Bases” and Their Countercations

In Suzuki-Miyaura reactions, anionic bases F(-) and OH(-) (used as is or generated from CO3(2-) in water) play multiple antagonistic roles. Two are positive: 1) formation of trans-[Pd(Ar)F(L)2] or trans-[Pd(Ar)-(L)2(OH)] (L = PPh3) that react with Ar'B(OH)2 in the rate-determining step (rds) transmetallation and 2) catalysis of the reductive elimination from intermediate trans-[Pd(Ar)(Ar')(L)2]. Two roles are negative: 1) formation of unreactive arylborates (or fluoroborates) and 2) complexation of the OH group of [Pd(Ar)(L)2(OH)] by the countercation of the base (Na(+), Cs(+), K(+)).

Kinetic and mechanistic investigation of the substitution reactions of four and five co-ordinated rhodium stibine complexes with a bulky phosphite

The substitution reaction of trans-[Rh(Cl)(CO)(SbPh3)2] (1) with tris(2,4-di-tert-butylphenyl)phosphite (2,4-TBPP) to form trans-[Rh(Cl)(CO)(2,4-TBPP)2] (4) in two consecutive steps has been investigated by UV-vis stopped-flow spectrophotometry. The experiments were performed in dichloromethane and in ethyl acetate, at 298 K and 268 K respectively for the first reaction step, and for the second reaction step over a temperature range from 278 to 313 K in both solvents. The first step is very fast (up to 1630 s(−1)) and on the limit of what is observable with the stopped-flow technique. Introduction of the five-coordinate complex trans-[Rh(Cl)(CO)(SbPh3)3] (2) in equilibrium with (1), by adding an excess SbPh3, led to a significant decrease in overall reaction rate for the formation of the intermediate trans-[Rh(Cl)(CO)(SbPh3)2(2,4-TBPP)] (3). Activation parameters for the second substitution reaction, in which 3 is converted to 4, has been determined as ΔH‡ = 22.85 ± 0.17 and 28.38 ± 0.10 kJ mol(−1) and ΔS‡ = −144.7 ± 0.6 and −100.9 ± 0.4 J mol(−1) K(−1) for CH2Cl2 and EtOAc respectively, supporting an associative pathway. A strongly coordinating solvent promotes both reactions. In all reaction steps a strong tendency for stibines to promote 5-coordinated, fairly stable intermediates is manifested.

Kinetic Data for the Transmetalation/Reductive Elimination in Palladium‐Catalyzed Suzuki–Miyaura Reactions: Unexpected Triple Role of Hydroxide Ions Used as Base

The mechanism of the reaction of trans-[ArPdX(PPh(3))(2)] (Ar=p-Z-C(6)H(4); Z=CN, F, H; X=I, Br, Cl) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4); Z'=CN, H, OMe) has been established in DMF in the presence of the base OH(-) in the context of real palladium-catalyzed Suzuki-Miyaura reactions. The formation of the cross-coupling product ArAr' and [Pd(0)(PPh(3))(3)] has been followed through the application of electrochemical techniques. Kinetic data have been obtained for the first time, with determination of the observed rate constant, k(obs), of the overall reaction. trans-[ArPdX(PPh(3))(2)] is not reactive in the absence of the base. The base OH(-) plays three roles. It favors the reaction: 1) by formation of trans-[ArPd(OH)(PPh(3))(2)], a key complex which, in contrast to trans-[ArPdX(PPh(3))(2)], reacts with Ar'B(OH)(2) (rate-determining transmetalation), and 2) by unexpected promotion of the reductive elimination from the intermediate trans-[ArPdAr'(PPh(3))(2)], which generates ArAr' and a Pd(0) species. Conversely, the base OH(-) disfavors the reaction by formation of the unreactive anionic Ar'B(OH)(3)(-). As a consequence of these antagonistic effects of OH(-), the overall reactivity is controlled by the concentration of OH(-) and passes through a maximum as the concentration of OH(-) is increased. Therefore, the base favors the rate-determining transmetalation and unexpectedly also the reductive elimination.

The trans influence in the modulation of platinum anticancer agent biology: the effect of nitrite leaving group on aquation, reactions with S-nucleophiles and DNA binding of dinuclear and trinuclear compounds.

To examine the effect of leaving group and trans influence on the general reactivity of polynuclear platinum antitumor agents we investigated substitution of the chloride leaving groups with nitrite ion, which forms strong bonds to Pt. It was of interest to explore whether nitrite could be used to modulate biological properties of these agents, in particular the deactivating reactions that occur on reaction with S-nucleophiles, involving loss of the linking diamine under the trans influence of sulfur. Reported herein is a study of the synthesis, aquation, DNA binding and reactions with glutathione (GSH), methionine (Met) and acetylmethione (AcMet) of nitrito derivatives of di- and trinuclear platinum antitumor compounds: [{trans-PtNO(2)(NH(3))(2)}(2)(mu-NH(2)(CH(2))(6)NH(2))](NO(3))(2) (1-NO(2)) and [{trans-PtNO(2)(NH(3))(2)}(2)(mu-trans-Pt(NH(3))(2){NH(2)(CH(2))(6)NH(2)}(2))](NO(3))(4) (1'-NO(2)). {(1)H,(15)N}-HSQC NMR studies revealed that 1-NO(2) is inert to aquation reactions, even after prolonged incubation at physiological pH. Monitoring of the interaction of 1-NO(2) with the duplex 5'-d(ATATGTACATAT)(2) (I) showed only unreacted complex, consistent with activation by aquation being a requirement for covalent DNA binding. The reaction of 1-NO(2) with GSH was studied by (1)H, (195)Pt, (15)N and {(1)H,(15)N}-HSQC NMR spectroscopy. For the parent dichlorido compounds (1 and 1') substitution of chloride by GS(-) leads to drug degradation involving liberation of the diamine linker. While the same final products trans-[Pt(SG)(2)(NH(3))(2)] (5) and trans-[{Pt(SG)(NH(3))(2)}(2)-mu-SG] (6) are formed, different mechanisms are involved, consistent with the trans influence NO(2)(-) > Cl(-); the half-life is slightly longer for 1-NO(2) (1.8 h) compared with 1 (1.3 h). Identification of the intermediate trans-[Pt(NH(3))(2)(NO(2))(SG)] (4) shows that the nitrito group remains coordinated while the linker amine is substituted by coordination of GS(-), and then trans labilization of the nitrito group occurs leading to 5 and 6. Reaction of the trinuclear 1'-NO(2) with GSH follows essentially the same reaction pathway. Reaction of 1-NO(2) with Met and AcMet is much slower and only 20 % liberated amine was observed after reaction with Met for 24 h at 37 degrees C. The final product from reaction with AcMet is trans-[Pt(NH(3))(2)(NO(2))(AcMet)], as in this case coordination of the S-nucleophile does not lead to trans labilization of the nitrito group.

10.1007/s11449-008-3002-9

The geometric symmetry group of the internal dynamics of (H2O)2 and (D2O)2 dimers taking into account two nonrigid motions with the lowest energy barriers is obtained. The motions are the inversion motion (or the acceptor switching) and the exchange motion of the acceptor and donor via an intermediate trans con-figuration. On this basis, a rigorous model for the description of such motions in the ground electronic-vibrational state is constructed. The model leads to a simple algebraic scheme for the calculation of both the positions of levels in the energy spectrum and the intensities of transitions between them. The classification of stationary states that takes into account the geometry of inversion and exchange motions is considered.

Synthesis and Structure of Novel RuII - N≡C - Me Complexes and their Activity Towards Nitrile Hydrolysis: An Examination of Ligand Effects

The synthesis and isolation of new RuII–acetonitrile complexes, of general formula trans,fac-[Ru(bpea)(B)(MeCN)](BF4)2 (bpea = N,N-bis(2-pyridylmethyl)ethylamine; B = bpy, 2,2′-bipyridine, 4; B = dppe, 1,2-bis(diphenylphosphino)ethane, 5), together with a synthetic intermediate trans,fac-[Ru(NO3)(bpea)(dppe)](BF4), 6, are described. Ru(bpea)Cl3, 1, is used as the starting material for the synthesis of all complexes 2–6 presented in this paper, which are characterized by analytical, spectroscopic (IR, UV/Vis, 1D and 2D NMR), and electrochemical techniques (cyclic voltammetry). Furthermore, complexes 4, 5, and 6 have also been characterized in the solid state by single crystal X-ray diffraction analysis. Their structures show a distorted octahedral geometry where the bpea ligand binds in a facial mode, the bidentate ligands bpy and dppe bind in a chelate manner, and finally the MeCN or the NO3 – ligand occupy the sixth position of the octahedral Ru metal centre. The kinetics of the basic hydrolysis of the coordinated MeCN ligand for complexes 4 and 5 and for the related complex [Ru(phen)(MeCN)([9]aneS3)](BF4)2, 7, which contains the 1,4,7-trithiacyclonane ligand ([9]aneS3) and 1,10-phenanthroline (phen) is also described. Second-order rate constants for acetonitrile hydrolysis measured at 25°C of k = 1.01 × 10–3 M–1 s–1 for 4, 1.08 × 10–4 M–1 s–1 for 5, and 6.8 × 10–3 M–1 s–1 for 7, have been obtained through UV-vis spectroscopy. Activation parameters have also been determined over the temperature range 25.0–45.0°C and agree with a mechanism that involves an associative rate-determining step. Finally the electronic and steric influence of the auxiliary ligands on this reaction for the above and related complexes is discussed.

Direct Observations of the Metal−Ligand Bifunctional Addition Step in an Enantioselective Ketone Hydrogenation

The catalytic intermediate trans-[Ru((R)-BINAP)(H)2((R,R)-dpen)] (1) reacted on mixing with acetophenone in THF at -80 degrees C under approximately 2 atm H2 to generate the alkoxide trans-Ru((R)-BINAP)(H)((Ph)(Me)CHO)((R,R)-dpen) (6). Contrary to expectations, free Ru-amide and 1-phenylethanol were not the immediate products of this addition reaction. The addition reaction was reversible in THF. 2-Propanol prevents racemization of the alcohol product in THF solvent.

Internal dynamics of (H2O)2 and (D2O)2 dimers: II. Effective operators of physical quantities with account of inversion and exchange motions

Using the symmetry group chain methods, a model of the internal dynamics of (H2O)2 and (D2O)2 dimers that takes into account their real geometries and two nonrigid motions with the lowest energy barriers is constructed. The motions are the inversion motion (or the acceptor switching) and the exchange motion of the acceptor and donor via an intermediate trans configuration. The model rigorously describes the interactions of various motions and leads to a simple algebraic scheme of calculation of both the positions of the levels in the energy spectrum and the intensities of transitions between them. It is important that the correctness of the model is limited only by the trueness of the choice of the symmetry of the internal dynamics.