The equivalence of the methyl groups in puckered 3,3-dimethyl oxetane

The spectroscopic study of molecules with large amplitude vibrations have led to reconsider the concept of molecular structure. Sometimes identifying definite bond lengths and angles is not enough to reproduce the experimental data so one must have information on the large amplitude molecular vibration potential energy function and dynamics. 3,3-dimethyloxetane (DMO) has non-planar ring equilibrium configuration and a double minimum potential function for ring-puckering with a barrier of 47 cm$^{-1}$.\footnote{ J. A. Duckett, T. L. Smithson, and H. Wieser, \emph{J. Mol. Spectrosc. } \textbf{1978}, \textit{69} , 159\emph{J. Mol. Struct. } \textbf{1979}, \textit{56}, 157}$^,$\footnote{ J. C. L\'{o}pez, A. G. Lesarri, R. M. Villamañán and J. L. Alonso, \emph{J. Mol. Spectrosc. } \textbf{1990}, \textit{141}, 231} The observation of endocyclic $^{13}$C and $^{18}$O monosubstituted isotopologues\footnote{ R. S\'{a}nchez, S. Blanco, A. Lesarri, J. C. L\'{o}pez and J. L. Alonso, \emph{Phys. Chem. Chem. Phys. } \textbf{2005}, \textit{7}, 1157} allow to conclude that the ring is puckered. However an interesting feature was observed for the $^{13}$C substitutions at the methyl carbon atoms. While two different axial and equatorial $^{13}$C-methyl groups spectra are predicted from a rigid non-planar ring DMO model, only one species was found. The observed rotational transitions appear at a frequency close to the average of the frequencies predicted for each isotopologue. The observed lines have the same intensity as that found for the $^{13}$C$_\alpha$ isotopomer and double that that found for the $^{13}$C$_\beta$ isotopomer.$^c$ This behaviour evidences that the two methyl groups of DMO are equivalent as could be expected for a planar ring. In this work we show how consideration of the potential function and the path for ring puckering motion to calculate the proper kinetic energy terms allow to reproduce the experimental results. \textit{Ab initio} computations at the CCSD/6-311++G(d,p) level, tested on related systems, have been done for this purpose

Characterization of microsolvated crown ethers from broadband rotational spectroscopy

Since they were first synthetized, crown ethers have been extensively used in organometallic chemistry due to their unparalleled binding selectivity with alkali metal cations. From a structural point of view, crown ethers are heterocycles containing oxygen and/or other heteroatoms, although the most common ones are formed from ethylene oxide unit. Crown ethers are conventionally seen as being hydrophilic inside and hydrophobic outside when the structures found for the metal cation complexes are considered. However, crown ethers are extremely flexible and in isolation may present a variety of stable conformations\footnote{F. G\'{a}mez, B. Mart\'{i}nez-Haya, S. Blanco,J. C. L\`{o}pez and J. L. Alonso, \emph{Phys. Chem. Chem. Phys.} \textbf{2014}, \textit{14} 12912-12918} so that their structure may be easily adapted in presence of a strong ligand as an alkali metal cation minimize the energy of the resulting complex. Water can be considered a soft ligand which interacts with crown ethers through moderate hydrogen bonds. It is thus interesting to investigate which conformers are selected by water to form complexes, the preferred interaction sites and the possible conformational changes due to the presence of one or more water molecules. Previous studies identified microsolvated crown ethers but in all cases with a chromophore group attached to the structure.\footnote{V. A. Shubert, C.W. M\"{u}ller and T. Zwier, \emph{J. Phys. Chem. A} \textbf{2009}, \textit{113} 8067-8079} Here we present a broadband rotational spectroscopy study of microsolvated crown ethers produced in a pulsed molecular jet expansion. Several 1:1 and 1:2 crown ether:water aggregates are presented for 12-crown-4, 15-crown-5 and 18-crown-6. Unambiguous identification of the structures has been achieved using isotopic substitution within the water unit. The subtle changes induced in the structures of the crown ether monomer upon complexation and the hydrogen-bonding network that hold them together will be also discussed

The formamide2-H2O complex: Structure and hydrogen bond cooperative effects

The adduct formamide$_2$-H$_2$0 has been detected in a supersonic expansion and its rotational spectra in the 5-13 GHz frequency region characterized by narrow-band molecular beam Fourier transform microwave spectroscopy (MB-FTMW). The spectrum shows the hyperfine structure due to the presence of two $^{14}$N-nuclei. This hyperfine structure has been analyzed and the determined quadrupole coupling constants together with the rotational constants have been a key for the identification of the adduct structure on the light of ab initio computations. The rotational parameters are consistent with the formation of a three body cycle thanks to the double proton acceptor/proton donor character of both formamide and water. The low value of the planar moment of inertia \textit{P}$_{cc}$ indicates that the heavy atom skeleton of the cluster is essentially planar. A detailed analysis of the results reveals the subtle effects of hydrogen bond cooperative effects in this system

Effects of typhoons on gelatinous carnivore zooplankton off Northern Taiwan

We examine the effect of typhoons, as potential drivers of nutrient pulse events, on gelatinous carnivore zooplankton. The period investigated spanned from 2007 to 2010, where seasonal abundance and taxonomic composition of the gelatinous zooplankton community was recorded off North Taiwan. Typhoon effects were assessed in the abundance, diversity and species richness of the gelatinous carnivore populations. Significant associations between typhoons and gelatinous carnivores were found in time delays varying from 3 to 25 days, but no association was identified for longer time intervals. Generally, a decrease in species richness occurred during the summer season, and this was accentuated in 2008, shortly after typhoons events. We hypothesize that typhoons might act as resource pulse triggers probably enhancing open niches for opportunistic carnivore zooplankton groups

Optical design of inhomogeneous media to perfectly focus scalar wave fields

A method to design isotropic inhomogeneous refractive index distribution is presented, in which the scalar wave field solutions propagate exactly on an eikonal function (i.e., remaining constant on the Geometrical Optics wavefronts). This method is applied to the design of ¿dipole lenses¿, which perfectly focus a scalar wave field emitted from a point source onto a point absorber, in both two and three dimensions. Also, the Maxwell fish-eye lens in two and three dimensions analyzed