THE ELECTRONIC SPECTRUM OF JET-COOLED H 2 PO, THE PROTOTYPICAL PHOSPHORYL FREE RADICAL Mohammed A. Gharaibeh and Dennis J. Clouthier Department of Chemistry, University of Kentucky, Lexington, KY Ricardo Tarroni Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna, Viale Risorgimento 4, Bologna, Italy

Background  Phosphoryl radicals are important intermediates in organic synthesis and polymerization processes. H 2 PO is one of the many products of the oxidation of phosphine  No previous studies of the electronic spectrum of the H 2 PO free radical  Microwave spectrum of this molecule was reported in 1996 [ T. Hirao, S. Saito, and H. Ozeki, J Chem Phys 105 (1996) ]  Studying the periodic trends in the heavy atom nitroxyl radicals which have the general formula H 2 XO where X is phosphorus or arsenic atom The dihydrophosphoryl radical (H 2 PO)

H 2 XO ground state structure (X=N,P and As) 15A CsCs CsCs C 2v Dihydronitroxyl radical Dihydroarsenyl radical Dihydrophosphoryl radical

π*π* π n a' a'' a' π*π* π n a'' a' π*π* π n a'' a' Ground State 1 st Excited State 2 nd Excited State T 0 = cm -1 T 0 = cm -1 Theoretical calculations / MO diagram CCSD-EOM / aug-cc-pV(T+d)Z X 2 A ~ A 2 A ~ B 2 A ~ ׳ ׳ ׳ ׳

Theoretical calculations / geometry Calculated molecular geometries π*π* π oop: O θ P H H ~ ~

Production of jet-cooled H 2 PO radical Pulsed valve Ring electrodes Reheat tube Vacuum (~10 -6 torr) psi of 5% PH 3 + 4% CO 2 in Ar HV psi of 5% PD 3 + 4% CO 2 in Ar H 2 PO D 2 PO

Low resolution LIF spectra ν 2 : PO stretch ν 3 : HPH sym bend ν 4 : HPH wag Observed isotope shift = 11.4 cm -1 Calculated Isotope shift = 17 cm -1

Emission spectra H 2 PO D 2 PO ν 2 : PO stretch ν 3 : HPH sym bend ν 4 : HPH wag

Vibrational constants H 2 PO D 2 PO ν 2 : PO stretch ν 3 : HPH sym bend ν 4 : HPH wag Calc = CCSD/aug-cc-pV(T+d)Z StateParameter (cm -1 ) H 2 POD 2 PO Obs.Calc.Obs.Calc. X 2 A′ ω 1 (PH stretch) (9) (7)1719 ω 2 (PO stretch) (5) (2)1173 ω 3 (HPH bend) (5) (6)821 ω 4 (HPH wag) 783.6(3) (5)600 B 2 A′ ω1 ω (4) (1)1693 ω2 ω (3) (1)983 ω3 ω (3) (2)825 ω4 ω (4) (5)692 ~ ~

Ground state : error = 0.0% Excited state : error = 2.3% Teller-Redlich product rule  : fundamental frequency B: B rotational constant M: total mass of the isotopologue m: atomic mass of H or D Frequency ratio Mass ratio

Strong a-type transitions Weak c-type transitions A′  A′ = A′ Rotational band types Transition moment vector c=x a=y

High resolution LIF spectrum of the 0 0 band 0

High resolution spectrum of H 2 PO

Rotational constants StateConstant (cm -1 )H 2 POD 2 PO X 2 A A5.2077(1)*2.6862(28) B (3)*0.5721(11) C (3)*0.5087(9) B 2 A A4.6186(13)2.4115(21) B0.5142(5)0.4705(12) C0.5036(5)0.4495(12) T0T (5) (5) * Microwave spectrum (T. Hirao, S. Saito and H. Ozeki, J. Chem. Phys. 105, 3450 (1996)) ε aa = cm -1 ″

 =102.6 ˚  =115.5 ˚ r =1.488Å r =1.429Å  =93.3˚  = ˚ r =1.671Å r =1.428Å Ground state Excited state oop=46.5 o oop=70.7 o Molecular geometry [1.407] [1.487] [114.6] [102.4] [1.417] [1.654] [105.8] [92.4] [66.9] [48.3] O θ P H H O θ P H H CCSD-EOM / aug-cc-pV(T+d)Z

Geometry: H 2 PO vs H 2 AsO State r (X  O) r (X  H)  (HXO)  (HXH)  (oop) T0T0 H 2 PO X 2 A B 2AB 2A Change H 2 AsO X 2 A B 2AB 2A Change

Summary The previously unknown electronic spectrum of the H 2 PO free radical has been identified in the nm region. The ground and the excited state a′ vibrational frequencies have been determined for H 2 PO and D 2 PO. The molecular structure of H 2 PO in the second excited state has been determined by rotational analysis of the 0 0 band. 0