= Solution
The natural mechanism is ionizing solvolysis: C-OTs cleavage gives a cyclopentyl cation/ion pair, capture by acetic acid gives the protonated acetate <ester>, and <deprotonation> gives product.
The \b[$1.15$] effect from H/D at the leaving-group carbon is normal alpha-secondary: that carbon develops trigonal <carbocation> character. The \b[$2.06$] effect from four neighboring labels is a combined beta-secondary effect. Adjacent C-H sigma bonds stabilize the developing empty p orbital by <hyperconjugation>; replacing H by D changes the vibrational stabilization and slows ionization. If the four contributions were equal and independent, each would be $2.06^{1/4}=1.198$.
None of those labeled C-H bonds is broken. The isotope effects support substantial <carbocation> character, although distinguishing a free cation from an intimate ion pair needs additional evidence.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-4-isotope-geometry.png]
{title=Retained-bond geometry and the sign of secondary isotope effects}
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