Table 4. Experimental and Calculated Δν_CO Values^a upon Protonation.

reactionb	exptc	calcdd
P1	22, 22	31, 30
P2e	78, 46	86, 37
P3	82, 79	89, 77
P4e	24, 21	27, 34
P5	19, 17	29, 29

^aValues given in cm^–1.

^bThe P1, P2, P3, P4, and P5 protonation reactions are defined in Figure 3.

^cExperimental data from ref (38).

^dFrequencies scaled by a factor of 0.9850.

^eThe second Δν_CO value is assigned to the semibridging CO.