m. douay and p.f. bemath- laser spectroscopy of canc and srnc

8/2/2019 M. Douay and P.F. Bemath- Laser spectroscopy of CaNC and SrNC

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Volume 174, number 3,4 CHEMICAL PHYSICS LETTER S 9 November 19 90

Laser spectroscopy of CaNC and SrNCM. Douay and P.F. Bemath Department of Chemistry. The U niversity fArizona, T ucson,A Z 85721 , U SAReceived 4 August 19 90; n final form 13 August 19 90

Low-resolution laser excitation and dispersed laser-induced fluorescence spectra of CaNC and SrNC were recorded. The laserexcitation spectra ofthe B X+-z *E* and x D-g 2E + ransitions of SrNC are consistent with a linear, isocyanide structure. Forboth Ca NC and SrNC, additional strong, non-resonant features occur to the red of the B X+-R E+ and A *H-g Z transitionsin the dispersed fluorescence spectra. Although these features remain unassigned, they might be due to emission from the iso-merit, linear cyanides, CaCN and SrCN. In this case, the excited state potential curves need to have a small barrier between tbecyanide and the isocyanide forms.

1. IntroductionPastemack and Dagdigian [ 1 ] first observed thealkaline earth monocyanide molecules. They noted

that Ca, Sr and B a vapors reacted with BrCN to ma keboth the metal bromides and the metal monocya-nides. More recently, Furio an d D agdigian [ 2 ] founda lower bound of 3.91 eV for the Ca-CN bondstrength from chemiluminescence measurements.The a b initio ca lculations of Bau schlicher, Langhoffand Partridge [3] predict that the alkaline earth monocyanides are, in fact, monoisocyanides. Theyfind linear M-NC structures for the entire family ofmolecules Be through Ba.

Gas-phase metal cyanides have long fascinatedchemists because of the possibility of nearly free in-ternal rotation of the CN group. Clementi, Kisten-macher and Popkie [4 ] called the ionic M-CN bondin LiCN polytopic because of the non-directionalcharacter of the bond. Experimentally, NaCN [5]and KCN [ 6,7] are found to be T-shaped, whileLiNC [ 8,9] has a linear isocyanide structure. Clearly,the geometry of the M+-C N- molecules is deter-

Curren t address: Labo ratoire de Spectroscopic des Molecu lesDiatomiques, Universitb des Sciences et Techniques de Lille,BPtiment P .5,5965 5 Villeneuve dAscq Cedex, France.2 Alfred P. Sloan Fellow, Camihe and Henry Dreyfbs Teacher-

Scholar.

mined by a balance between various subtle forces1101.Very recently W hitham et al. [ 111 observed CaNCin a supersonic jet expansion source by pulsed laserexcitation spectroscopy. They obtained a rotation-ally resolved spectrum of the 000-000 band of theA 2II-g 2C+ ransition of linear CaNC near 6 15 nm.These results inspired us to re-examine some old, low-resolution CaNC and SrNC spectra [ 121.

2. ExperimentalmethodsThe CaNC and SrNC were made by the reaction

of Ca and Sr vapors with CH,CN in a Broida oven[ 131. The experimental arrangemen t w as similar toour previous work with monovalent derivatives ofthe alkaline earth metals [ 14-231. The metals wereresistively heated in a n alum ina crucible and the va-por entrained in a flow of Ar carrier gas. The totalpressure in the oven was about 1.5 Torr with a fewmTorr of CH3CN.No reaction occurs between acetonitrile (CH$N )and the Ca or Sr vapors without the help of a broad-band (1 cm-) dye laser used to excite the PI-Sotransition of the metal atom. In a few experimentswe used ICN as the oxidant. The reaction with ICNproceeds without laser excitation of the m etal vaporbut with a much reduced production of the metal

23 0 000 9.261 4/90/$ 03.50 0 199 0 - Elsevier Science Publishers B.V. (North-Holland)


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Volume174, number 3,4 CHEMICAL PHYSICS LETTERS 9 November 1990monoisocyanide. Even when a laser was used to pro-mote the Ca or Sr reaction with ICN, the MNC sig-nal was at least a factor of two less than with CH ,CN.Because CH &N is less toxic than ICN and the spec-tra were similar with both o xidants, we routinely usedCH,CN.The CaCN and SrCN spectra are close to the cor-responding CaOH [24,25] and SrOH [ 26,271 spec-tra. It was therefore helpful to dry the CH$ N vaporby passing it through a CaCl, drying tube before itentered the oven.

A second broadband cw dye laser (Rhodamine110, Rhodamine 6G, DC M or pyridine 2 dyes) wasused to excite fluorescence from the CaNC or SrNCmolecules. The first laser, which was resonant withthe 3P,-S,, transition at 6573 A for Ca and 6892 Afor Sr, was always operated with DCM dye. Laserexcitation spectra of SrNC were recorded by scan-ning the second dye laser and detecting the total flu-orescence through a red-pass filter w hich blocked theatomic transition at 6892 A. No laser excitationspectra for CaNC were recorded because the CaNCtransitions occur just to the blue of the Ca 3P,-1S0transition.

Fo r both CaNC and SrNC dispersed fluorescencewere recorded by leaving futed the wavelength of bothlasers and scan ning a 0.64 m m onochromator. Themonochrom ator was operated w ith a GaAs photo-.multipier tube (RCA 31034) and photon countingelectronics. Unfortunately, the laser-induced fluo-rescence was nearly com pletely relaxed. Apart fromsome changes in the relative intensity of bands, allof the dispersed fluorescence spectra were very sim-

ilar, regardless of which ban d the laser was exciting.Both dc and ac detection (with a lock-in amplifier)were employed.

3. Results and discussionThe simplest spectrum to interpret is the laser ex-

citation spectrum of SrNC (fig. 1). There are threestrong peaks at 64186577 and 67 10 A correspond-ing to B X+-R 2Z, a 2113,2-% Z+ and x 211111,2-2 2 C+ transitions (fig. 1). The spin-orbit splittingin the x II state is 30 I cm-, very similar to the cor-responding splitting in SrNCO (293 cm- ) [211 andSrNNN (296 cm- ) [201. The value of the spin-or-bit coupling constant is consistent with linear struc-ture and the ab initio calculations of Bauschlicher,Langhoff and Partridge [ 31 suggest that SrNC islower in energy than SrCN in the groun d jT:22;f state.Unfortunately, these calculations [31 were for all ofthe alkaline earth monoisocyanides, BeNC throughBaNC, except SrNC.

The dispersed fluorescence spectrum is much moredifftcult to interpret (fig. 2 ) because of the presenceof additional red features at 6906,705O and 7224 A.These features are present only very weakly in thelaser excitation spectra (fig. 1) recorded with DCMand pyridine 2 dyes (not shown). It is initiallytempting to assign these features to a vibrationalprogression in the Sr-NC stretching mode, but theground state vibrational intervals w ould be 423,295and 342 cm-. These intervals are too erratic to bean unperturbed vibrational progression.

650 nm 660 670

Fig. . Laserexcitationpectrumf SrNC.231


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Volume 174, number $4 CHEMICAL PHYSICS LETTER S 9 November 1990

SrNC

I I I I550 670 BQO 710 In

Fig. 2. Dispersed fluorescence spectrum of SrNC . The laser wavelength, marked with an asterisk, is exciting the A ll,,2-ji *I? transi-tion. A definitive assignmen t is unav ailable for the strong red emission at 6 90 6 and 70 50 A.The negative-going feature at 6 892 A s theSr 3PI-LS,, tomic transition. At 6 892 Aboth he SrN C molecule and the S r atom 8 re emitting light, so the lock -in amplifier was overloaded.

Moreover, by assuming the CN ligand is a pointmass, a pseudodiatomic reduced mass c&nbe com-puted. Since CN- is often treated as a pseudohalidewith properties intermediate between F- and Cl-,one can empirically estimate a vibrational frequencyof 393 cm- for !&-NC nd 464 cm- for CaN C fromthe vibrational frequencies of SrF (502 cm-), SrCl(302 cm-), CaF (581 cm-) andCaCl(367 cm-)[27]. A similar calculation using SrNN N (3 16cm-) as the parent mo lecule predicts 377 cm- forSrNC and 449 cm- for CaNC from the vibrationalfrequency of CaNNN (396 cm- ). The predictedSrNC ground state vibrational frequency of 377-393cm- is not consistent with the observed intervals.The corresponding dispersed fluorescence spec-trum for CaN C (fig. 3) show s structure to the red ofthe A II-% %+ and B 2Z+-x 2Z transitions. Inthis case, the ,&-jt and &g transitions are com-pletely overlapped at 6 I73 A in our spectra. The laserexcitation spectra of the cold CaNC recorded byWhitham et al. [ 111 showed the A lIs,2-jt 2Zf and,&211,,2-ii Z+ transitions at 1626 8 cm- (6145 A)and 16191 cm- (6175 A), respectively. They didnot assign the B P-2 *C+ transition although it isprobably just to the blue.For CaNC the red emission occurs at 6326 and6478 A (fig. 3) and the intervals between the threepeaks are 392 and 370 cm-. These intervals are toosmall to be unperturbed ground state Ca-NC232

stretching frequencies, expected to be 449-46 4 cm-.Unfortunately, we were unable to record a good laserexcitation spectrum (see section 2), but like theSrNC, these red features would probably be rela-tively weak in the excitation spectrum.Another possible explanation for these unassign edred features is that they belong to ano ther m olecule.One possible molecule is, for example, CaNCCH,,and Whitham et al. [ 111 found some evidence fora heavier molecule in their spectrum. However,Wh itham et al. s [ 111 impurity spectrum lies to blueof the CaNC features. Molecules other than CaNCand S rNC are probably minor contributors to the ob-served spectra becau se we find the spectra essentiallyunchanged when ICN is used as an oxidant ratherthan CH&N.

The rapid relaxation which occurs in the excitedelectronic states of CaNC and SrNC is an imp ortantclue. In our extensive experience with monovalentalkaline earth d erivatives [ 14-231, the observationof completely non-resonant emission for a sm all tria-tomic molecule is unique. Most of the laser-inducedemission from the tetra-atomic SrNNN [20] wasresonant under o ur experimental conditions and thisallowed a rotational analysis to be carried out. Weobserved mainly non-resonant em ission for the largermembers of the monoalkoxide [ 141, mo nothiolate[28 ] and monoalkylamide [ 171 families (MOR,MSR and M NHR ), but not for small molecules such


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Volume 174 , number 3,4 CHEMICAL PHYSICS LETTERS 9 November 19 90

orbit coupling will be quenched in the A *ITstate asthe linear isocyanide structure bends. These com-plications should be viewed as a unique opportu-nity to study the dynamics of molecular motion inthe excited electronic states of CaCN and SrCN . In-deed, the ground state potential surfaces of LiNC,NaC N an K CN are often stud ied theoretically as ex-amp les of systems which display chaos (for ex-ample, see refs. [ 35-391).

AcknowledgementThis work was supported by the National Science

Foundation (CHE-8913785).

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m. douay and p.f. bemath- laser spectroscopy of canc and srnc

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