147
Quantum Compass of Migratory Birds
(Δg mechanism), or hyperfne interaction (HFI) mechanism including an electron-exchange interac
tion in the singlet hydrogen-bonded radical-ion pair (HFI-J mechanism) in a radical-pair intermedi
ate (Salikhov et al., 1984). Te magnetic feld efect due to the HFI-J mechanism is considered to be
particularly interesting and important from the viewpoint of mechanistic photochemistry because it
is expected when hydrogen or electron transfer between a photoexcited molecule and the hydrogen-
bonded species occurs to form an appropriate hydrogen-bonded radical pair or radical ion-pair inter
mediate in a solvent cage (Hata, 1985).
In 1976, for the frst time, Hata found this type of a photochemical magnetic feld efect due to the
HFI-J mechanism in the case of the photochemical isomerization of isoquinoline N-oxide in ethanol
(Hata, 1976). Hata (1976) investigated the photochemical reaction of isoquinoline N-oxide in ethanol
with or without magnetic feld up to 17 kg (1.7 T) and measured the chemical yield of lactam (isocarbo
styril). Te magnetic feld efects on the yield of lactam (isocarbostyril) in the photochemical reaction
of isoquinoline N-oxide in ethanol is shown by Hata (1976). Here, the chemical yield of lactam was
65%–68% below 5 kg (0.5 T), and decreased drastically to be ca. 52% at about 10 kg (1 T). Further increase
in the feld strength resulted in the recovery in the chemical yield to reach a constant value of ca. 65%.
Tus, the yield of lactam indicated a minimum value at about 10 kg (1 T). Tese results suggested that
magnetic feld could enhance intersystem crossing from the excited singlet spin state of isoquinoline
N-oxide at about 10 kg (1 T). In 1978, this new phenomenon was successfully interpreted in terms of
HFI-J mechanism assumed to be a transient intermediate of this reaction (Hata, 1978). Tese studies
were partially reported in preliminary form (Hata, 1976, 1978; Hata et al., 1979, 1983).
Hata (1985) presented a further detailed mechanism of the photochemical isomerization of isoquino
line N-oxide. Te magnetic feld dependence of the chemical yield of lactam 2 in the photochemical
reaction of isoquinoline N-oxide 1 is shown by Hata (1985). When the chemical reaction was carried out
with or without magnetic felds up to 1.6 T, the chemical yield of lactam 2 was measured. Te chemical
yield of lactam was ∼67% below 0.8 T and decreased drastically to be ∼52% at about 1 T. Further increase
in the feld strength resulted in the recovery in the chemical yield to reach a constant value of ∼67%. Te
conversion remained almost constant at ∼17%.
Te chemical yield of oxazepine 5 vs. magnetic feld strength in the photochemical reaction of 1 cya
noisoquinoline N-oxide 4 is presented by Hata (1985). Te results of the chemical yield of oxazepine 5
against the feld strength proved to be independent of an external magnetic feld. Here, also, the conver
sion remained almost constant at ∼30%.
As for the second example of the photochemical magnetic feld efects, Hata and Nishida (1985)
reported the photoinduced substitution reaction of 4-methyl-2-quinolinecarbonitrile in ethanol and
cyclohexane. External magnetic feld efects on the photosubstitution reaction (1→2) in ethanol are
shown by Hata and Nishida (1985). Here, 1, 4-methyl-2-quinolinecarbonitrile; 2, 2-(1-hydroxyethyl)
4-methylquinoline. Chemical yield of 2 vs. magnetic feld strength. [1] = 4.01 × 10−3 mol/dm3. (1) open
circles: [C5H8] = 0, (2) closed circles: [C5H8] = 3.0 × 10−1 mol/dm3. Te chemical yield of 2-(1-hydroxyethyl)
4-methylquinoline 2 is plotted as a function of the feld strength in the absence or presence of 1,3-pen
tadiene, where the conversion remained almost constant (20%–22%). In the absence of 1,3-pentadiene,
as shown by curve (a), the chemical yield of 2 was ca. 48% at the zero feld. However, it increased qua
dratically with an increase in the feld strength to be ca. 58% at about 1.5 T (the magnetic feld efect
due to Δg mechanism). Te chemical yield of 2 also showed a minimum (ca. 49%) at about 1.1 T (the
magnetic feld efect due to the HFI-J mechanism). Te Δg magnetic feld efect, as shown by curve (b),
disappeared completely upon the addition of 1,3-pentadiene, although the magnetic feld efect due to
the HFI-J mechanism was still observed. Tus, the chemical yield of 2 was ca. 58% at a magnetic feld
below 0.8 T, but it decreased steeply with an increase in the feld strength to become ca. 48% at about 1.1
T. Further increase in the feld strength resulted in the quadratic recovery in the chemical yield to reach
a constant value of ca. 58%. Te results explicitly indicate that the Δg or the HFI-J magnetic feld efect
observed in a photochemical reaction can be assigned to the feld dependence of the chemical yield of
the T1- or S1-born cage product.