ConspectusA decisive factor for obtaining high yields and selectivities in organic synthesis is the choice of proper solvent. Solvent selection often guided by intuitive understanding transition state–solvent interactions. However, quantum-mechanical tunneling can significantly contribute to chemical reactions, circumventing state thus depriving chemists their handle on reaction kinetics. In this Account, we aim provide rationales effects solvation reactions derived from experiments...

Alcohol solvents are significantly more polar than expected based on the measured H-bonding properties of monomeric alcohols in dilute solution.

The interactions between diphenylcarbene DPC and the halogen bond donors CF3I CF3Br were investigated using matrix isolation spectroscopy (IR, UV–vis, EPR) in combination with QM QM/MM calculations. Both CF3X form very strong complexes singlet state of DPC, but only weakly interact triplet DPC. This results a switching spin becoming more stable than complexes. forms second complex (type II) that is thermodynamically slightly stable. Calculations predict this DPC···I distance shorter F3C···I...

Abstract As an experimental test of the theoretical prediction that heavy‐atom tunneling is involved in degenerate Cope rearrangement semibullvalenes at cryogenic temperatures, monodeuterated 1,5‐dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using matrix isolation technique. predicted, less thermodynamically stable isotopomer rearranges temperatures dark to more one, while broadband irradiation above 2000 cm −1 results equilibration isotopomeric ratio....

Abstract Carbenes are among the few metal‐free molecules that able to activate molecular hydrogen. Whereas triplet carbenes have been shown insert into H 2 through a two‐step mechanism at low temperature is assisted by quantum mechanical tunneling (QMT), singlet in concerted reactions with considerable activation barriers, and thus unreactive towards cryogenic temperatures. Here we show 1‐azulenylcarbene ground state readily inserts , QMT governs insertion both D . This first example shows...

1-Azulenylcarbene was synthesized by photolysis of 1-azulenyldiazomethane in argon or neon matrices at 3-10 K. The highly polar singlet carbene is only metastable and undergoes a tunneling rearrangement to 8-methylene-bicyclo[5.3.0]deca-1,3,5,6,9-pentaene. After substitution the 4 8 positions with deuterium, completely inhibited. This indicates very large kinetic isotope effect, as expected for reaction.

Abstract As an experimental test of the theoretical prediction that heavy‐atom tunneling is involved in degenerate Cope rearrangement semibullvalenes at cryogenic temperatures, monodeuterated 1,5‐dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using matrix isolation technique. predicted, less thermodynamically stable isotopomer rearranges temperatures dark to more one, while broadband irradiation above 2000 cm −1 results equilibration isotopomeric ratio....

5-Methoxy-2H-benzazirine was prepared via irradiation of the corresponding phenyl azide, isolated in an argon matrix at cryogenic temperatures. It undergoes ring expansion to ketenimine dark T < 30 K despite a calculated activation barrier 4.9 kcal mol-1 [B3LYP/6-311++G(d,p)]. Since this rearrangement proceeds with rate constant order 10-4 s-1, exhibiting only shallow temperature dependence, results are interpreted terms heavy-atom tunneling. Of four isomeric benzazirines resulting from...

Abstract Hydrogen bonding is a central concept in chemistry and biochemistry, so it continues to attract intense study. Here, we examine hydrogen the H 2 S dimer, comparison with well-studied water unprecedented detail. We record mass-selected IR spectrum of dimer superfluid helium nanodroplets. are able resolve rotational substructure each three distinct bands and, based on it, assign these vibration-rotation-tunneling transitions single intramolecular vibration. With use high-level...

ABSTRACT Interstellar complex organic molecules (iCOMs) have been identified in different interstellar environments including star forming regions as well cold dense molecular clouds. Laboratory studies show that iCOMs can be formed either gas phase or the solid state, on icy grains, from ‘non-energetic’ (atom-addition/abstraction) energetic (UV-photon, particle bombardments) processes. In this contribution, using a new experimental approach mixing matrix isolation technique, mass...

Abstract 4‐Oxocyclohexa‐2,5‐dienylidene is a highly reactive triplet ground state carbene that hydrogenated in solid H 2 , HD, and D at temperatures as low 3 K. The mechanism of the insertion into dihydrogen was investigated by IR EPR spectroscopy kinetic studies. or atoms were observed products reaction with respectively, whereas HD produces exclusively atoms. hydrogenation shows very large isotope effect remarkable selectivity, expected for tunneling reaction. experiments, therefore,...

Abstract Carbenes are among the few metal‐free molecules that able to activate molecular hydrogen. Whereas triplet carbenes have been shown insert into H 2 through a two‐step mechanism at low temperature is assisted by quantum mechanical tunneling (QMT), singlet in concerted reactions with considerable activation barriers, and thus unreactive towards cryogenic temperatures. Here we show 1‐azulenylcarbene ground state readily inserts , QMT governs insertion both D . This first example shows...

High-throughput UV–vis titrations in combination with chemical double-mutant cycles (DMCs) have been used to study the competition of a polar solvent for formation intramolecular H-bonds. Twenty-four different zinc porphyrin–pyridine complexes were investigated mixtures toluene and phenol. DMCs determine effective molarities (EM) phenol–amide H-bonds as function composition. The values EM increase by an order magnitude increasing concentrations more solvent, Phenol solvates amide groups on...

Abstract Four isomeric azulenylcarbenes were synthesized in argon matrices by photolysis of the corresponding diazo precursors, and photochemistry these carbenes was studied. The their rearranged products characterized IR, UV/Vis, EPR spectroscopy, experimental data compared to results from DFT calculations. While 2‐, 5‐ 6‐azulenylcarbene show triplet ground states, 1‐azulenylcarbene exhibits a singlet state, accord with theoretical predictions. rearrangements give access number unusual C 11...

The ring-opening of a highly strained cyclopropene to carbene proceeds via heavy-atom tunneling. This rearrangement is accelerated in the presence H 2 O, ICF 3 or BF , resulting novel Lewis-acid catalyzed tunneling reaction.

The elusive diradicals α,2- and α,4-didehydrotoluene 1 3, have been generated by photolysis of matrix-isolated 2-iodobenzyl iodide 7 4-iodobenzyl 8, respectively. Diradical 3 could also be synthesized flash vacuum thermolysis with subsequent trapping the products in argon at 5 K. their triplet ground states were characterized electron paramagnetic resonance spectroscopy. accordance previous theoretical calculations for these species.

Chirality switching of self-assembled molecular structures is potential interest for designing functional materials but restricted by the strong interaction between embedded molecules. Here, we report on an unusual approach based reversible chirality changes oligomers using variable-temperature scanning tunneling microscopy supported quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped Ag(111). At 130 K, a temperature far lower than...

o-Tolylmethylene 1 is a metastable triplet carbene that rearranges to o-xylylene 2 even at temperatures as low 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double (ENDOR) spectroscopical techniques were used identify two conformers of (anti syn) in noble gas matrices frozen organic solutions. Conformer-specific kinetic measurements revealed the rate constants for rearrangements anti syn are very similar. However, orbital alignment conformer...

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF3I has been studied by matrix isolation spectroscopy. carbenes were produced photolysis corresponding diazo compounds, matrix-isolated in argon doped 1% at 3 K. Bimolecular reactions between induced annealing these matrices to 25–30 K allow for diffusion trapped species. Instead classical halogen-bonded complexes, form complexes which iodine atom is shared carbene center CF3 group....

A new sextet ground state molecule, the 2,4,6‐trichloro‐1,3‐dinitrenophenoxyl radical, was obtained upon UV photolysis of corresponding diazido precursor in argon at 5 K. The molecule is formed stepwise with triplet nitrene, quartet nitreno and quintet dinitrene as intermediates that were detected by EPR spectroscopy. IR UV–Vis measurements only allowed us to observe main product, dinitrene. coupling between two nitrene centers oxygen centered radical results zero‐field splitting (zfs)...

The structural arrangements of α-keto acid complexes hold significant interest across various fields chemistry such as enzyme modeling, drug design, or polymer blending. Herein, we report mass-selective infrared (IR) spectra pyruvic monomers and dimers in the range 1720–1820 cm–1 recorded helium nanodroplets at 0.37 K. monomer features IR bands 1807.1 1734.5 cm–1, which are assigned to carboxylic ketonic C═O stretching vibrations, respectively. Furthermore, generated inside characterized by...

Here, we report the results of an IR spectroscopy study on heteroclusters H

Alkali metal amides are highly reactive reagents that broadly applied as strong bases in organic synthesis. Here, we use a combined helium nanodroplet IR spectroscopic and theoretical (DFT calculation) study to show the reaction of model compound lithium hexamethyldisilazide (LiHMDS) with water is close barrierless even at ultra-cold conditions. Upon complex formation dimeric (LiHMDS)2 nanodroplets nano-reactors (0.37 K) observed product (LiOH)2(HMDS)2. This can be rationalized aggregation...