Dynamic NMR (DNMR) spectroscopy of [R 1 C(R 2 )SO 2 R 3 ]Li (R 1 , R 2 = alkyl, phenyl; R 3 = Ph, tBu, adamantyl, CEt 3 ) in [D 8 ]THF has shown that the S-tBu, S-adamantyl, and S-CEt 3 derivatives have a significantly higher enantiomerization barrier than their S-Ph analogues. C α -S bond rotation is most likely the rate-determining step of the enantiomerization of the salts bearing a bulky group at the S atom and two substituents at the C α atom.
Abstract. For almost two decades, the airborne Fast In-situ Stratospheric Hygrometer (FISH) has stood for accurate and precise measurements of total water mixing ratios (WMR, gas phase + evaporated ice) in the upper troposphere and lower stratosphere (UT/LS). Here, we present a comprehensive review of the measurement technique (Lyman-α photofragment fluorescence), calibration procedure, accuracy and reliability of FISH. Crucial for FISH measurement quality is the regular calibration to a water vapor reference, namely the commercial frost-point hygrometer DP30. In the frame of this work this frost-point hygrometer is compared to German and British traceable metrological water standards and its accuracy is found to be 2–4 %. Overall, in the range from 4 to 1000 ppmv, the total accuracy of FISH was found to be 6–8 %, as stated in previous publications. For lower mixing ratios down to 1 ppmv, the uncertainty reaches a lower limit of 0.3 ppmv. For specific, non-atmospheric conditions, as set in experiments at the AIDA chamber – namely mixing ratios below 10 and above 100 ppmv in combination with high- and low-pressure conditions – the need to apply a modified FISH calibration evaluation has been identified. The new evaluation improves the agreement of FISH with other hygrometers to ± 10 % accuracy in the respective mixing ratio ranges. Furthermore, a quality check procedure for high total water measurements in cirrus clouds at high pressures (400–500 hPa) is introduced. The performance of FISH in the field is assessed by reviewing intercomparisons of FISH water vapor data with other in situ and remote sensing hygrometers over the last two decades. We find that the agreement of FISH with the other hygrometers has improved over that time span from overall up to ± 30 % or more to about ± 5–20 % @ < 10 ppmv and to ± 0–15 % @ > 10 ppmv. As presented here, the robust and continuous calibration and operation procedures of the FISH instrument over the last two decades establish the position of FISH as one of the core instruments for in situ observations of water vapor in the UT/LS.
[1] The long-term changes of the stratospheric mixing ratio of CH 4 over the period of 1978-2003 are derived from balloon-borne data of H 2 , CH 4 and N 2 O. The data were obtained by collecting whole air samples and subsequent gas chromatographic analyses. To eliminate the short-term variability attributed to dynamical processes, the N 2 O mixing ratio is used as a proxy for altitude. A correlation analysis for the individual years is applied and the CH 4 mixing ratios are interpolated to four different levels of N 2 O, corresponding to altitudes of approximately 17, 23, 26 and 30 km at midlatitudes. For the investigated period of 1978 to 2003 we find increases at the four levels of 207 ± 32 ppb, 159 ± 21 ppb, 140 ± 34 ppb and 111 ± 60 ppb, respectively. The CH 4 trend has slowed down in recent years and is best fitted by a second-order polynomial. The increase of CH 4 can account for only 25-34% of the increase in stratospheric H 2 O of 1%/yr over the last decades as derived from previous studies. The simultaneously measured time series of stratospheric H 2 mixing ratios shows that the contribution of stratospheric H 2 to the H 2 O trend in the period 1988-2003 is minor.
The structures of the norbornenyl and norbornyl sulfones exo‐5, endo‐5 and endo‐6 have been determined experimentally, by X‐ray analysis, and theoretically by ab initio calculations (HF/6–31+G*). X‐ray crystal structure analyses of the lithiated allylic norbornenyl and norbornyl sulfones endo‐3/ent‐endo‐3·2diglyme and endo‐4/ent‐endo‐4·2diglyme revealed dimeric O–Li contact ion pairs devoid of C–Li bonds. The anions of endo‐3/ent‐endo‐3·2diglyme and endo‐4/ent‐endo‐4·2diglyme adopt both the endo conformation (C2–S) and are characterized by in the exo direction pyramidalized anionic C atoms. The degree of pyramidalization of the C2 atom of 3 is higher than that of 4. Ab initio optimizations (HF/6–31+G*) of the structures of the anions of methylenenorbornene I and methylenenorbornane II resulted in local minima featuring non‐planar C2 atoms which are pyramidalized in the exo direction in both cases, but to different degrees. In both cases cryoscopy of 3 and 4 in THF at –108.5 °C revealed approximately 1:1 mixtures of monomers and dimers. The sulfones exo‐5, endo‐5, exo‐6 and endo‐6 as well as the lithiosulfones 3 and 4 were studied by NMR spectroscopy. 1H‐NMR (400 MHz), 13C‐NMR (100 MHz) and 6Li‐NMR (44 MHz) spectroscopy of 3 and 4 at –100 °C in [D]8THF revealed in each case only one set of signals, independent of the configuration of the starting sulfones. This indicates in both cases that attainment of both the monomer‐dimer and the endo/exo equilibria of 3 and 4 is fast on the NMR time scale. According to 6Li{1H}‐ and 1H{1H}‐NOE experiments of 3 and 4 the monomeric and dimeric species endo‐3 and endo‐4, having endo anions, seem to be preferred in THF solution. Ab initio calculations of the anions of 3 and 4 resulted in structures endo‐3(–Li+), exo‐3(–Li+), endo‐4(–Li+) and exo‐4(–Li+) (HF/6–31+G*), whose atomic point charges were calculated by the method of Kollman et al. The C2 atoms of endo‐3(–Li+) and endo‐4(–Li+) are pyramidalized in the exo direction whereas the C2 atoms of exo‐3(–Li+) and exo‐4(–Li+) are pyramidalized in the endo direction. According to the calculations, the endo anions are more stable than the exo anions. There is good agreement between the optimized structures of the free anions and the experimentally determined structures of the anions of the contact ion pairs in the crystal. Reactions of 3 and 4 with DX, MeI, EtI, nPrI and nHeI occurred at the C2 atom under the selective formation of the corresponding endosulfones endo‐8a–e and endo‐9a–e, respectively, in all cases. Thus, an earlier report on the selective formation of the exosulfone exo‐9b in the reaction of 4 with MeI has to be revised. Product ratios were independent of the configuration of the starting sulfones and varied with the nature of the electrophile. Selectivities were highest in the case of the norbornyl species 4. Reaction of 3 with PhCHO occurred at the α position (C2) to afford the alcohols endo‐8f and exo‐8f (88:12) as single diastereomers and at higher temperatures at the γ position (C8), whereas reaction of ...
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