1. 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 40506-0055 Ricardo Tarroni Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy

2. 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)

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

4. 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 = 11200 cm -1 T 0 = 26700 cm -1 Theoretical calculations / MO diagram CCSD-EOM / aug-cc-pV(T+d)Z X 2 A ~ A 2 A ~ B 2 A ~ ׳ ׳ ׳ ׳

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

6. Production of jet-cooled H 2 PO radical Pulsed valve Ring electrodes Reheat tube Vacuum (~10 -6 torr) 40-100 psi of 5% PH 3 + 4% CO 2 in Ar HV 40-100 psi of 5% PD 3 + 4% CO 2 in Ar H 2 PO D 2 PO

7. 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

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

9. 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) 2221.4(9)24031576.5(7)1719 ω 2 (PO stretch) 1163.4(5)11901141.2(2)1173 ω 3 (HPH bend) 1085.9(5)1135795.7(6)821 ω 4 (HPH wag) 783.6(3)788595.5(5)600 B 2 A′ ω1 ω1 2321.9(4)23611601.5(1)1693 ω2 ω2 790.7(3)843904.3(1)983 ω3 ω3 1073.7(3)1132781.0(2)825 ω4 ω4 1027.0(4)1076672.47(5)692 ~ ~

10. Ground state 0.388 : 0.388 error = 0.0% Excited state 0.376 : 0.385 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

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

12. High resolution LIF spectrum of the 0 0 band 0

13. High resolution spectrum of H 2 PO

14. Rotational constants StateConstant (cm -1 )H 2 POD 2 PO X 2 A A5.2077(1)*2.6862(28) B0.6341107(3)*0.5721(11) C0.5944851(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) T0T0 26583.862(5)26572.154(5) * Microwave spectrum (T. Hirao, S. Saito and H. Ozeki, J. Chem. Phys. 105, 3450 (1996)) ε aa = -0.044 cm -1 ″

15.  =102.6 ˚  =115.5 ˚ r =1.488Å r =1.429Å  =93.3˚  =105.68 ˚ 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

16. 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 1.4881.429115.5102.646.50 B 2AB 2A 1.6711.428105.6893.366.826583.86 Change 0.183 -0.001-9.82-9.3 20.3 H 2 AsO X 2 A 1.6721.513106.6101.863.10 B 2AB 2A 1.8061.525103.193.470.719613.54 Change 0.134 0.012-3.5-8.4 7.6

17. Summary The previously unknown electronic spectrum of the H 2 PO free radical has been identified in the 407 - 337 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