JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry Val. 73A, No.2, Marc h- April 1969 A Table of Rotational Constants of Symmetric Top Molecules Givi n9 Rise to Microwave Spectra* Matthew S. Lojko Environmental Science Services Administration, Boulder, Colorado 80302 and Yardley Beers National Bureau of Standards, Boulder Colorado 80302 (December 23, 1968) This paper li sts, in order of increasin g value, the " B" rot ati onal constants of most of the linear and symmetri c top molecules whic h have been observed by mi crowave s pectroscopy. Also are listed the microwave spectral lines which have been observed for the asymmetric tops water and formalde ­ hyde. These d ata are useful for making a quick selection of a molecul e whi ch has a s pectral line close to some previously selected freque ncy. Key words: Asymmetric, linear, and symmetric top molecules; "B" rotationa l constant s; formal­ dehyde; mi c rowave spectra; s pectral lines; wa ter. I ( Nowadays the re are a number of actual and poten­ give n in eq (l) it is possible to get a very good approxi­ tial si tuations in whic h one would lik e to select a mation of th e frequencies of nearly all of the rotational mol ecul e to have a spectral line close to some pre­ lin es of these molecules. The freque ncy l viously specifi e d frequency_ For one thing, the trans i­ tions of li ght linear or symme tric top molecul es, whic h j = 2Bj, (1 ) I li e in almost but not exact harmonic relationship, are where } is any integer, whi ch is th e quantum number used to tune up millimeter wave spectrographs_ For gi ving the total angular mome ntum (not in cluding t another, there is a great interest in de vi sing ex peri­ nuclear spin ) of th e uppe r state giving rise to the transi­ ments, many involving lasers, in which a spectral line, tion. In table 1 , values of B are given for only one iso­ or one of the harmonics of its freque ncy, nearly coin­ topic s pecies of each molecule, generally the most cides with a s pectral line of another subs tance_ In abundant one_ The values for other isotopic species addition there might be practical applications, such as may be easily determined from the fact that B is in­ in a communications system, in which it may be de­ versely proportional to the reduced mass of the mole­ sired to build a molecular frequency standard with cule. Also listed in table 1 are the common names of its fundamental close to some previously chosen the molecules to aid in referring to handbooks giving fr eque ncy. physical properties and to the catalogs of chemical As an aid to making such a selection, herewith in companies. table 1 is presented, in order of increasing value, the The values of B given in table 1 were obtained from "B" rotational co nstants of most of the linear and the same punched co mputer cards whi c h serv ed as the symmetric top molecules which are listed in "Micro­ basis of the " Mi c rowave Spectral Tables." A program wav e Spectral Tables," National Bureau of Standards, was written for selectin g the values in accordance with Monograph 70.' From the si mple well-known formula the present philosophy, arrangi ng the values in ascend- "'Co ntribution (If NalilJtwl Bureau of Sta ndards and Environme ntal Science Servi ces Volume IV : PoJ ya tomic Molecul es Without Internal Rotation, M. S. Cord , J. O. Petersen. Administration. IHJI s ubject tu copyright. M . S. Loj ku. and R. H. Haas. October 1968. I These Tables ma y bt~ nbtained through the S uperintende nt of Doc um ent s, U.S. Gov­ Volume V: S pectral Line Listing, by M. S. Curd. M. S. Lojko, and J. D. Petersen. June e rnme nt Printing Offi ce. W as hington. D.C. 20402. 1968. The follo win g vulum es have been published: V(J lurne I: Diatomic Molecul es. by P. F. Wacker. M. Mizushima. J. D. Petersen. a nd Publicati on of the remaining volume is expected shortly: J. R. Ballard. I)eccmbcr 15. 1964. Volume II : Line Strengths of ASY IlHnelric Rotors. by P. F. Wacker and M. R. Pratt o. Volume III : 1\)lyalomic Molecules With Internal Hotation, P. F. Wacker. M. S. Cord. December 15. 1964. D. G. Burkhard. J. D. Petersen. and H. F. Kukol. 233 ing order, and printing out the res ult. In this tabulation, of the Offi ce of Standard Reference Data of the Na­ it has not been practical to di stinguish between ground tional Bureau of Standards when it became establi s hed. vibrational state (Bo) and equilibrium state (Be) values. It is likely that in due tim e supplements to the present ~ In most cases, however, for polyatomic molecules the series will be publis hed unde r sponsorship of the valu es given pertain to th e ground vibrational state Office. while for- diatomic molecul es they pertain to the equilib­ Volume V, which li sts the measured frequencies of rium state. Howe ver, the difference between values of spectral lin es in orde r of freque ncy, is also useful in these quantities for th e same molec ul e is probably giving direct ans wers to questions of th e type described smaller than th e e rrors associated with eq (1) whic h in th e o pe nin g paragraph above. Indeed most of the result fmm the neglect of s uc h e ffects as centrifugal time it may be more useful th a n table 1 and eq (1). di stortion and hyperfin e effects. However. the present material is useful in providing Accordingly, it is a ppropriate to give some general some answers not containe d in Volume V, and there· di scussion of the tables. The present seri es of volumes fore availability of the present material is highly are based upon a thorough search of the literature of desirable. One reason is that Volume V contains only , mi crowave s pectroscopy. The c ut-off date of the lines th at have ac tually been observed and meas ured search differs sli ghtly from vo lume to volume but is accurately. However, there exist ma ny lines wh ic h approximately 1961 for all of the m. In addition, through are observabl e but which have not been observed, the cooperation of numerous investi gators, much and there are others whic h have been observed but unpublished data and data published in sources not not measured accurately and not published. In addi­ widely available have been included. In a number of tion , Volume V contains very fe w entries above 300 cases whe re important data afte r the official c ut-off GHz and none above 500 GHz. At the same time, the date happened to be conveniently available, they were developme nt of submillimeter lasers has stimulated included, but in clusion of post-cut-off data was de­ a great interest in the fre quencies of spectral lines termined by conve ni ence and nothing else. A critic al up to a few thousand megahertz. The materi al give n selection was made, and the selected data on freque n­ he re can be used to give good approximations of the cies and molecular constants were transcribed upon frequencies of many spectral lines which are involved punched cards. The cards were checked against the in either of these two categories. Once eq (1) has been ori ginal sources. Except for a few nonstandard situa­ used to select a molecular transition , th e frequency ti ons, all final print-o ut was made direct from the cards may be computed more exactly by including such by use of s uitable programs. The present series of effects as centrifugal di stortion and hyperfine inter­ volumes were carried out in accorda nce with a plan actions by methods contained in textbooks but beyond de veloped by Paul W. Wacker. It received s ponsorship the scope of thi s paper. T AB LE 1. "8 " Rotational constants of symmetric top and linear molecules B(MHz) Molecul e name Isotopi c species a 708.3579 Cesium Iodide Cs'""I 127 725.9 I·Bro mo·Bicyclo(2,2,2)Octa ne q2H , 3Br79 729.8 Be nze ne C hromium Tricarbonyl Q2H.Cr;2(CI20'6J:, 788.0 Methyl Me rc uric Iodid e CI2 H3Hgb l b 817.510 Thallium Iodide TJ20' 1'27 891.69 l -C hloro-3 ,3- Dimet h yl-l -But yne (C" H"hCI2C'2= C'"CpS 984.3166 Rubidium Iodide RbH' I'27 996.4 Phosphorus Tribro mide pb Br~9 1081.343 Cesium Bromide Cs''''' Br'') 1096.42 Trime thyli odosil ane (C" H3J:,S i2H I 127 1104.95 Indium Iodide In 11 :i T1 2i 1142.86 Me thyl Me rc uric Bromide C '2H" Hg'!1RBr79 1247.61 Tribro mometha ne C'2 HBrX9 1259.02 l -Iodo propyne H"CI2CI2=CI21' 27 1259.25 Nic ke l Cyclopentadie nyl N itrosyl Q2H, N14Ni'HO'6 1297.409 Thallium Bromide TI2°'Br") 1369.70 1,1 ,1-TriAuoro-2-But yn e C'2 H"C I 2=C I 2C '2F~9 1402.64 Thiophosphoryl C hloride S'''P'''C I ~5 1424.840 Rubidium Bromide RbH' Br7!1 1461.95 Trirnethylbromosilane (C " H3hSi2H Br79 1467.98 Cyclopentadie nyl Thallium Q 2H5 TJ2"" 1497.