The obtained spectroscopic constants are reported inTable 2 No mb-type transitions were observed in accordancewith the Cs symmetry of the isomer The Cs symmetry
makes the two 35Cl atoms of the parent species equivalentto each other and correspondingly their quadrupole cou-pling constants have the same values As mentioned abovethe ma-type spectrum for the 35Cl37Cl isotopologue has alsobeen assigned and measured in natural abundance Its spec-troscopic parameters are listed in the second column ofTable 2 In this case the 35Cl and 37Cl nuclei are differentfrom each other and in addition the geometrical symmetryof the complex is destroyed so that two different sets ofquadrupole coupling constants are required Because a small-er number of lines were measured for this isotopologue thed1 and d2 centrifugal distortion parameters were fixed at thevalues of the parent species whilst the off-diagonal quadru-polar coupling constants cab and cac were fixed at the theo-retical values Disappointingly we did not succeed in meas-uring at least four transitions of the 37Cl37Cl isotopologuebecause its abundance was only about 10 of that of theparent species
A comparison of the experimental spectroscopic parame-ters with the theoretical values for the two conformations inTable 1 leads to a straightforward assignment of the ob-served spectrum to isomer I which is stabilized by two CHmiddotmiddotmiddotClC and one CHmiddotmiddotmiddotFC WHBs
No lines belonging to isomer II were identified despitethe very small difference in complexation energy This resultcould be due to conformational relaxation to the moststable isomer upon supersonic expansion Indeed it hasbeen shown that this kind of relaxation takes place easily ifthe interconversion barrier is smaller than 2kT[22]
The Cs configuration of the observed isomer of CH2F2ndashCH2Cl2 is shown in Figure 2 From the rotational constantsof the two isotopologues it is possible to calculate the sub-stitution coordinates rs
[23] of the Cl atom in the principalaxes of the parent species The obtained values are shown inTable 3 and are compared with the values of a partial r0structure
The partial r0 structure was obtained by adjusting threestructural parameters (RC1C4 aH7C4middotmiddotmiddotC1 andaF2C1middotmiddotmiddotC4) whilst keeping the remaining parameters fixedto their ab initio values (preserving the Cs symmetry) to re-produce the six experimental rotational constants The ob-tained parameters are reported in Table 4 and compared tothe ab initio values From this partial r0 structure a (theangle between the Cl-C-Cl and bc planes) and the lengths ofthe three WHBs were derived (Table 4) The full ab initiogeometry is available in the Supporting Information
A [MHz] 2663073(3)[a] 2604320(3)B [MHz] 9584016(2) 9511963(1)C [MHz] 7851948(1) 7754507(1)
DJ [kHz] 07171(7) 0710(1)DJK [kHz] 10813(6) 988(7)d1 [kHz] 01604(7) ACHTUNGTRENNUNG[01604][b]d2 [kHz] 00613(4) ACHTUNGTRENNUNG[00613][b]caaACHTUNGTRENNUNG(35Cl) [MHz] 37399(5) 3717(3)cbbccc (35Cl) [MHz] 4368(2) 4234(3)cab ACHTUNGTRENNUNG(35Cl) [MHz] 900(7) 1116[c]
4971(6) 5003(2)caaACHTUNGTRENNUNG(37Cl) [MHz] 2962(2)cbbccc ACHTUNGTRENNUNG(37Cl) [MHz] 3544(2)cab ACHTUNGTRENNUNG(37Cl) [MHz] 752[c]cac ACHTUNGTRENNUNG(37Cl) [MHz] 619[c]
[a] Uncertainties (in parentheses) are standard deviations expressed inunits of the last digit [b] The numbers in parentheses are fixed at thevalues obtained for the parent species [c] Fixed at the values obtainedfrom the theoretical calculations [d] Number of lines in the fit [e] Stan-dard deviation of the fit
Figure 2 The observed isomer of CH2F2ndashCH2Cl2 with atom numberingand the positions of the principal axes a denotes the angle between thebisector of the Cl-C-Cl valence angle and the bc plane
[a] 1404 1473 0239[a] Calculated from the r0 structure in Table 4 the sign of the b coordi-nates depends on the specific Cl atom owing to the symmetry
Chem Asian J 2014 9 1032 ndash 1038 2014 Wiley-VCH Verlag GmbHampCo KGaA Weinheim1035
Quadrupole Coupling Constants
The nuclear quadrupole hyperfine structure considerablycomplicates the rotational spectrum but its analysis can pro-vide useful information on the structure and internal dynam-ics in the complex This analysis would become possible ifthe principal nuclear quadrupole tensor could be deter-mined because for a hyperfine nuclei terminal to a bondthis tensor is typically oriented to within 18 of the directionthe relevant bond axis[24] The only three non-zero compo-nents of the principal hyperfine tensor cg=eQqg (g=x yz) can be obtained from the quadrupole tensor that is ex-perimentally determined in the principal inertial axes Thelatter tensor consists of diagonal quadrupole coupling con-stants caa cbb and ccc and off-diagonal cab cbc and cac con-stants Diagonalization of the corresponding 33 matrix re-sults in three principal hyperfine tensor components czz cxxand cyy which are conventionally labeled in such a way thatczz describes the molecular-field gradient around the axisclose to the bond axis which is in this case the CCl axis
We performed the transformation by using the QDIAGprogram available on the PROSPE website[24ndash26] which alsoprovided the rotation angles between the two axis systemsOne of the more useful of these angles qzb allows an esti-mate of the aCl-C-Cl valence angle from the relation aCl-C-Cl= (1802qzb)8 The quadrupole asymmetry parameterh= (cxxcyy)czz is also evaluated These parameters arecompared in Table 5 with those for the CH2Cl2 monomerThe differences do not appear to be significant thus suggest-ing that vibrational averaging in the cluster has little effecton the chlorine nuclear quadrupole hyperfine splitting
Therefore we can use the quadrupole orientation to cal-culate how much the Cl-C-Cl plane in the complex is tiltedaway from the bc plane This result is quantified by usingthe tilt angle (a) as defined in Figure 2 The hyperfine esti-mate of this angle as obtained from QDIAG a=106(1)8 isclose to the values from the ab initio geometry and from thepartial r0 structure thereby providing additional independ-ent confirmation of the determined structure
Dissociation Energy
The intermolecular stretching motion that leads to the disso-ciation appears to be almost parallel to the a axis of thecomplex By assuming that such a motion is separated fromthe other molecular vibrations it is possible withina pseudo-diatomic approximation to estimate the stretchingforce constant according to Equation (2)[27] where m is thepseudo-diatomic reduced mass and RCM (3771 ) is the dis-tance between the centers of mass of the two subunits B Cand DJ are the spectroscopic parameters reported in Table 2
ks frac14 16p4 ethmRCMTHORN2 frac124B4thorn4C4ethBCTHORN2ethBthornCTHORN2=ethhDJTHORN eth2THORN
If then it is assumed that the intermolecular separation forthis kind of complex can be described by a LennardndashJones-type potential approximation the dissociation energy can beevaluated from Equation (3)[28] thus leading to ED=
76 kJmol1 which is very close to the ab initio value(ED(BSSE)=70 kJmol1)
ED frac14 1=72 ks RCM2 eth3THORN
In Table 6 we compare the dissociation energy of CH2F2ndashCH2Cl2 to those of some related adducts among freon mole-cules From these data it appears difficult to get a rule onthe relative strengths of the CHmiddotmiddotmiddotCl and CHmiddotmiddotmiddotF interac-tions
Conclusions
The microwave spectrum of CH2F2ndashCH2Cl2 represents anunprecedented investigation of this type of intermolecularcomplex by rotational spectroscopy This cluster whichexists as a combination of two asymmetric molecules con-tains two heavy quadrupolar nuclei (35Cl or 37Cl) with highnuclear spin quantum numbers and large electric nuclearquadrupole moments The consequent complex hyperfine
Table 4 Partial r0 and re structures of CH2F2ndashCH2Cl2
Fitted parametersRC1C4 [] aH7C4middotmiddotmiddotC1 [8] aF2C1middotmiddotmiddotC4 [8]
r0 3755(1)[a] 625(1) 557(1)re 3751 635 504
Derived parametersRF2H7 [] RCl5H9 [] a [8][b]
r0 2489(2) 3147(2) 118(1)re 2421 3139 138
[a] Uncertainties (in parentheses) are expressed in units of the last digit[b] The angle between the Cl-C-Cl plane and the bc inertial plane
Table 5 The principal quadrupole tensors h q and aCl-C-Cl forCH2Cl2 and CH2F2ndashCH2Cl2
CH2Cl2[a] CH2F2ndashCH2Cl2
czz [MHz] 754(2) 7416(6)cxx [MHz] 334(2) 3531(8)cyy [MHz] 399414(2) 3885(7)h[b] 0060(3) 0048(1)q [8][c] 3343(5) 336(1)aCl-C-Cl [8][d] 1131 1127
[a] see Ref [15] [b] h= (cxxcyy)czz [c] This angle corresponds to qza forCH2Cl2 and qzb for CH2F2ndashCH2Cl2 [d] Estimate obtained from 1802qcompared with aCl-C-Cl=11188 from the structural analysis of the mo-nomer (see Ref [15])
Table 6 Binding energies of the investigated dimers of freons
WHBs ED [kJmol1] Reference
CH3FmiddotmiddotmiddotCHF3 three CHmiddotmiddotmiddotFC 53 [13]CH2F2middotmiddotmiddotCH2F2 three CHmiddotmiddotmiddotFC 87 [10]CH2ClFmiddotmiddotmiddotFHC=CH2 one CHmiddotmiddotmiddotClC
one CH2middotmiddotmiddotFC87 [14]
CH2F2middotmiddotmiddotCH2Cl2 two CHmiddotmiddotmiddotClCone CHmiddotmiddotmiddotFC
76 this work
Chem Asian J 2014 9 1032 ndash 1038 2014 Wiley-VCH Verlag GmbHampCo KGaA Weinheim1036
wwwchemasianjorg Walther Caminati et al
structure of each transition has been successfully analyzedand interpreted in terms of five or ten quadrupole couplingparameters depending on the equivalence (35Cl35Cl) or not(35Cl37Cl) of the two Cl atoms The complex has a plane ofsymmetry with two equivalent Cl atoms as confirmed bythe key available experimental data 1) the existence of onlyone 35Cl37Cl isotopologue with 23 intensity of that of theparent species 2) the values of the Cl substitution coordi-nates and 3) the number and values of the quadrupole cou-pling constants
The detection of isomer I in which the two subunits arelinked to each other through two CHmiddotmiddotmiddotClC and one CHmiddotmiddotmiddotFC WHBs rather than isomer II in which the units arelinked through two CHmiddotmiddotmiddotFC and one CHmiddotmiddotmiddotClC interac-tions suggests that CHmiddotmiddotmiddotClC is a stronger linkage thanCHmiddotmiddotmiddotFC
Within a pseudo-diatomic approximation in which thetwo subunits are considered to be rigid in the angular coor-dinates the dissociation energy of this complex has been es-timated to be of similar value to that of the dimer of CH2F2
Experimental Section
The molecular clusters were generated in a supersonic expansion underoptimized conditions for the formation of the adduct Details of the Four-ier-transform microwave spectrometer[29] (COBRA-type[30]) which coversthe range 65ndash18 GHz have been described previously[31]
A gaseous mixture of about 1 CH2F2 and CH2Cl2 (commercial samplesused without further purification) in He at a stagnation pressure of about05 MPa was expanded through a solenoid valve (General Valve Series 9nozzle diameter 05 mm) into the FabryndashProt cavity The line positionswere determined after Fourier transformation of the time-domain signalwith 8k data points recorded at sampling intervals of 100 ns Each rota-tional transition appears as a doublet owing to the Doppler Effect Theline position was calculated as the arithmetic mean of the frequencies ofthe Doppler components The estimated accuracy of the frequency meas-urements was better than 3 kHz Lines that were separated by more than7 kHz were resolvable
Acknowledgements
We acknowledge the Italian MIUR (PRIN project 2010ERFKXL_001)and the University of Bologna (RFO) for financial support QG alsothanks the China Scholarships Council (CSC) for financial supportMVL gratefully acknowledges a FPI grant from MICINN and ZK ac-knowledges a grant from the Polish National Science Centre (decisionnumber DEC201102AST200298)
[1] E Arunan G R Desiraju R A Klein J Sadlej S Scheiner I Al-korta D C Clary R H Crabtree J J Dannenberg P HobzalH G Kjaergaard A C Legon B Mennucci D J Nesbitt PureAppl Chem 2011 83 1619ndash1636
[2] For example see The weak hydrogen bond in structural chemistryand biology Vol IX (Eds G R Desiraju T Steiner) IUCr mono-graphs on crystallography Oxford University Press Oxford 2001
[3] For example see a) J-M Lehn Angew Chem Int Ed Engl 198827 89ndash112 Angew Chem 1988 100 91ndash116 b) J-M LehnAngew Chem Int Ed Engl 1990 29 1304ndash1319 Angew Chem1990 102 1347ndash1362
[4] For example see T Steiner Angew Chem Int Ed 2002 41 48ndash76 Angew Chem 2002 114 50ndash80
[5] For example see S N Delanoye W A Herrebout B J Van derVeken J Am Chem Soc 2002 124 11854ndash11855
[6] For example see W Caminati J-U Grabow Microwave spectrosco-py Molecular systems in Frontiers of molecular spectroscopy (Ed JLaane) Elsevier Amsterdam 2008 Chapter 15 pp 455ndash552
[7] Y Tatamitani B Liu J Shimada T Ogata P Ottaviani A MarisW Caminati J L Alonso J Am Chem Soc 2002 124 2739ndash2743
[8] For example see a) J L Alonso S Antolnez S Blanco A Lesar-ri J C Lpez W Caminati J Am Chem Soc 2004 126 3244ndash3249 b) Q Gou G Feng L Evangelisti M Vallejo Lpez A Le-sarri E J Cocinero W Caminati Phys Chem Chem Phys 201315 6714ndash6718 c) S Blanco J C Lpez A Lesarri W CaminatiJ L Alonso ChemPhysChem 2004 5 1779ndash1782 d) L B FaveroB M Giuliano S Melandri A Maris P Ottaviani B Velino WCaminati J Phys Chem A 2005 109 7402ndash7404 e) P OttavianiW Caminati L B Favero S Blanco J C Lpez J L AlonsoChem Eur J 2006 12 915ndash920
[9] a) L B Favero B M Giuliano A Maris S Melandri P OttavianiB Velino W Caminati Chem Eur J 2010 16 1761ndash1764 b) MVallejo-Lpez L Spada Q Gou A Lesarri E J Cocinero W Ca-minati Chem Phys Lett 2014 591 216ndash219 c) L Spada Q GouM Vallejo-Lpez A Lesarri E J Cocinero W Caminati PhysChem Chem Phys 2014 16 2149ndash2153
[10] W Caminati S Melandri P Moreschini P G Favero AngewChem Int Ed 1999 38 2924ndash2925 Angew Chem 1999 111 3105ndash3107
[11] S Blanco S Melandri P Ottaviani W Caminati J Am Chem Soc2007 129 2700ndash2703
[12] G Feng L Evangelisti I Cacelli L Carbonaro G Prampolini WCaminati Chem Commun 2014 50 171ndash173
[13] W Caminati J C Lpez J L Alonso J-U Grabow Angew ChemInt Ed 2005 44 3840ndash3844 Angew Chem 2005 117 3908ndash3912
[14] C L Christenholz D A Obenchain S A Peebles R A Peebles JMol Spectrosc 2012 280 61ndash67
[15] Z Kisiel J Kosarzewski L Pszczlkowski Acta Phys Polon A1997 92 507ndash516
[16] Y Niide H Tanaka I Ohkoshi J Mol Spectrosc 1990 139 11ndash29[17] a) Z Kisiel L Pszczlkowski W Caminati P G Favero J Chem
Phys 1996 105 1778ndash1785 b) Z Kisiel L Pszczlkowski L BFavero W Caminati J Mol Spectrosc 1998 189 283ndash290
[18] Gaussian 03 (Revision B01) M J Frisch G W Trucks H B Schle-gel G E Scuseria M A Robb J R Cheeseman J A Montgom-ery Jr T Vreven K N Kudin J C Burant J M Millam S SIyengar J Tomasi V Barone B Mennucci M Cossi G ScalmaniN Rega G A Petersson H Nakatsuji M Hada M Ehara KToyota R Fukuda J Hasegawa M Ishida T Nakajima Y HondaO Kitao H Nakai M Klene X Li J E Knox H P HratchianJ B Cross C Adamo J Jaramillo R Gomperts R E StratmannO Yazyev A J Austin R Cammi C Pomelli J W Ochterski P YAyala K Morokuma G A Voth P Salvador J J DannenbergV G Zakrzewski S Dapprich A D Daniels M C Strain OFarkas D K Malick A D Rabuck K Raghavachari J B Fores-man J V Ortiz Q Cui A G Baboul S Clifford J CioslowskiB B Stefanov G Liu A Liashenko P Piskorz I Komaromi R LMartin D J Fox T Keith M A Al-Laham C Y Peng A Na-nayakkara M Challacombe P M W Gill B Johnson W ChenM W Wong C Gonzalez J A Pople Gaussian Inc PittsburghPA 2003
[19] S F Boys F Bernardi Mol Phys 1970 19 553ndash566[20] M H Pickett J Mol Spectrosc 1991 148 371ndash377[21] J K G Watson in Vibrational Spectra and structure Vol 6 (Ed
J R Durig) Elsevier New YorkAmsterdam 1977 pp 1ndash89[22] For example see R S Ruoff T D Klots T Emilson H S Gutow-
ski J Chem Phys 1990 93 3142ndash3150[23] J Kraitchman Am J Phys 1953 21 17ndash25[24] Z Kisiel E Bialkowska-Jaworska L Pszczolkowski J Chem Phys
1998 109 10263ndash10272
Chem Asian J 2014 9 1032 ndash 1038 2014 Wiley-VCH Verlag GmbHampCo KGaA Weinheim1037
wwwchemasianjorg Walther Caminati et al
[25] Z Kisiel in Spectroscopy from Space (Eds J Demaison K SarkaE A Cohen) Kluwer Academic Publishers Dordrecht 2001pp 91ndash106
[26] Z Kisiel PROSPEndashPrograms for ROtational SPEctroscopy avail-able at httpwwwifpanedupl~kisielprospehtm
[27] a) D J Millen Can J Chem 1985 63 1477ndash1479 b) W G ReadE J Campbell G Henderson J Chem Phys 1983 78 3501ndash3508
[28] S E Novick S J Harris K C Janda W Klemperer Can J Phys1975 53 2007ndash2015
[29] T J Balle W H Flygare Rev Sci Instrum 1981 52 33ndash45
[30] a) J-U Grabow W Stahl Z Naturforsch A 1990 45 1043ndash1044b) J-U Grabow doctoral thesis Christian-Albrechts-Universitt zuKiel Kiel 1992 c) J-U Grabow W Stahl H Dreizler Rev Sci Ins-trum 1996 67 4072ndash4084 d) J-U Grabow HabilitationsschriftUniversitt Hannover Hannover 2004
[31] W Caminati A Millemaggi J L Alonso A Lesarri J C Lopez SMata Chem Phys Lett 2004 392 1ndash6
Received December 28 2013Revised January 21 2014
Published online February 26 2014
Chem Asian J 2014 9 1032 ndash 1038 2014 Wiley-VCH Verlag GmbHampCo KGaA Weinheim1038
wwwchemasianjorg Walther Caminati et al